'Ta' (iii) was the Japanese attempt to destroy the US lodgement around Cape Torokina on Bougainville island in the Solomon islands group (8/25 March 1944).

This extensive lodgement had been established, secured and then enlarged in the Empress Augusta Bay area, on and to the north-west of Cape Torokina, by Major General Allen H. Turnage’s 3^rd Marine Division of Lieutenant General Alexander A. Vandegrift’s (later Major General Roy S. Geiger’s) I Amphibious Corps in the highly successful and little opposed 'Cherryblossom' during November 1943 to complete the eastern portion of the 'Elkton' plan, otherwise known as 'Cartwheel'.

As soon as the initial beach-head area had been secured, Major General Robert S. Beightler’s 37^th Division moved into it from 8 November, and the two US divisions then expanded the beach-head into a lodgement containing a fighter airstrip, a bomber base and a good forward anchorage, the strategic reasoning of Admiral William F. Halsey and the staff of his South Pacific Area command being that the lodgement would contain the Japanese on the island (comprising the 40,000 men of Lieutenant General Masatane Kanda’s 6^th Division and Lieutenant General Yashushi Sakai’s 17^th Division of Lieutenant General Harukichi Hyakutake’s 17^th Army, as well as some 20,000 naval troops) and additionally provide a base area from which Rabaul, the primary Japanese base area on New Britain island, could be neutralised and the Japanese lines of communication from Rabaul to New Guinea and the Solomon islands group interdicted by US air and naval power.

'Ta' (iii) was the second but first fully planned Japanese counter-offensive against the Empress Augusta Bay lodgement. The initial response of the Japanese army to the 'Cherryblossom' landings was both slow and ineffective, largely as a result of the fact that the army was convinced that the landings in Empress Augusta Bay were a diversion designed to aid the 'real' assault which, the Japanese were sure, would be committed farther to the north against Buka island. As a consequence, most of the Japanese reinforcements were sent to Buka island rather than against the US lodgement in Empress Augusta Bay. Even so, the Japanese made a counter-landing between 04.00 and 06.00 on 7 November, when 850 men of the 17^th Division from Rabaul were landed from 21 landing barges convoyed by four destroyers. By the time the US forces appreciated what was taking place, the Japanese troops were already coming ashore. The landing took place at Koromokina Lagoon, which was so close to the northern edge of the perimeter round Cape Torokina that it isolated one US Marine Corps company, which had to be evacuated by two tank landing craft. A major artillery barrage on the morning 8 November then shattered the Japanese force, inflicting more than 300 casualties, and marines moving into the area found that the survivors had fled into the jungle. A dive-bomber attack just outside the marine perimeter on 9 November caught the Japanese troops as they tried to return to the area. This attack inflicted further heavy casualties, and the Japanese fled the area for good.

The marines recovered documents indicating that Lieutenant General Hitoshi Imamura, commanding the 8^th Area Army at Rabaul, planned to land 3,000 men in three echelons close to the marine perimeter. These were to infiltrate the perimeter while two battalions of the 23^rd Regiment, part of Major General Sun Iwasa’s 6^th Brigade of the 6^th Division, moved overland from the south-eastern part of Bougainville and assembled just to the east of the Piva river before falling on the eastern part of the marine perimeter. However, the destruction of the first landing echelon dissuaded Imamura from any further attempts at counter-landing close to the US perimeter.

The 23^rd Regiment meanwhile undertook probing attacks on the marine perimeter on 5 November, encountering a road block manned by elements of the 2^nd Marine Raider Battalion on the Mission Trail, which was the main trail from the Cape Torokina area to the east. On the night of 5/6 November, the marines drove off two Japanese preliminary attacks. Colonel Edward A. Craig estimated that this was just a preparatory action, and therefore brought up more marine raiders to reinforce the road block. When the Japanese attacked during the afternoon of 7 November, in an undertaking timed to coincide with the counter-landing at Koromokina Lagoon, the marines were therefore fully prepared and drove the Japanese back to Piva village with the aid of heavy fire from their supporting mortars. Early on the morning of 8 November, Iwasa renewed the attack, preceded by a four-hour mortar bombardment, with a strength of two complete battalions. The Americans brought up men of the 3^rd Marine Raider Battalion, as well as a number of light tanks, to protect the flanks of the marine elements already engaged, and the Japanese frontal attacks were beaten back.

Turnage came to the conclusion that the Japanese lodged on the trail represented a threat to the lodgement’s airstrips and had to be removed, and the US assault started on the morning of 9 November. The Japanese had sited several machine guns in tactical useful positions and were attempting an attack of their own, and there developed a bloody stalemate. Marine firepower eventually proved too much for the Japanese, who retreated first to and then through Piva village in the early and middle parts of the afternoon, and the marines took possession of the vital intersection of the Piva and Numa Numa Trails. Here the marines halted and dug in around Piva village.

During the period 8/13 November the Japanese undertook three major air raids against the shipping off Cape Torokina, making hugely exaggerated claims of sunken US battleships and carriers. In fact, the worst damage was two bomb hits and a torpedo hit on the light cruiser Birmingham, which was little impaired in its fighting capacity, and a torpedo hit on one of the engine rooms of light cruiser Denver, which forced the warship to retire at low speed. These results cost the Japanese 121 out of the 173 carrier aircraft committed to Rabaul on 1 November, together with 86 aircrew. Similar casualties were suffered by the 11^th Air Fleet. The Japanese raids did not prevent the arrival of Beightler’s 37^th Division from 9 November, the day on which Geiger assumed command of the I Amphibious Corps from Vandegrift, and four days later Geiger also assumed command of the entire Torokina beach-head area from Rear Admiral Theodore S. Wilkinson. By this time, the perimeter extended along some 7,000 yards (216400 m) of beach front and had a circumference of about 16,000 yards (14630 m).

After the battle of the Piva Trail and the capture of Piva village, in which the marines lost 115 men killed or wounded and the Japanese about 1,070 men killed, a small party of 'Seabee' construction battalion personnel, with a covering infantry patrol, undertook a reconnaissance well forward of the current perimeter in search of a site suitable for airfield construction. Finding a suitable area to the north-east of perimeter, in the area to the west of the junction of the Numa Numa and East-West Trails, the 'Seabees' set about preparing for the construction of landing strips. The senior 'Seabee' officer returned to the marine perimeter a day before the combat patrol, which made contact with a Japanese patrol on 10 November. Further patrols were undertaken up the Piva Trail, beyond the coconut grove near its junction with the East-West Trail, but these did not establish contact with the Japanese. As a result of the enormous difficulties in movement and supply through the swamps of the area, it was impossible to push forward the lodgement’s perimeter far enough to include the proposed airfield site, however, and it was therefore decided to establish a strong outpost, capable of sustaining itself until the line of the perimeter could be advanced to it, at the junction of the Numa Numa and East-West Trails, in order to avoid a fight for the airfield site should the Japanese occupy it first.

On the afternoon of 12 November, Turnage ordered the 21^st Marines to despatch a patrol of company strength up the Numa Numa Trail at 06.30 on 13 November. The patrol was to move up the Numa Numa Trail to its junction with the East-West Trail and then reconnoitre each trail for a distance of about 1,000 yards (915 m) with a view of establishing a strong outpost in the vicinity in the near future with Company E, 21^st Marines under the command of Captain Sidney Altman.

Additional orders from headquarters then ordained that the patrol should be increased in strength to two companies, with a suitable command group and an artillery forward observer team, and the task of the patrol was modified to include the immediate establishment of the planned outpost at the junction of the East-West and Numa Numa Trails. Given the clear significance of the envisaged task, Colonel Evans Ames, the commander of the 21^st Marines, asked for authorisation, which was granted, to send the entire 2/21^st Marines under the command of Lieutenant Colonel Eustace Smoak. This battalion’s Company E set out at 06.30 on 13 November, and moved to an assembly area behind the 9^th Marines, where it was to wait for the rest of the battalion to join it.

The rest of the battalion was issued with rations, water and ammunition, and awaited the arrival of the artillery forward observer party. At 07.30, and without the rest of the battalion, Company E was ordered to move along the Numa Numa Trail and embark on the establishment of the outpost, and at 08.00 the company started to advance along the Numa Numa Trail without incident. When the company had reached a point about 200 yards (185 m) to the south of its objective, it was struck at 11.05 by heavy fire from a Japanese ambush position. A runner was sent to Smoak, informing him of the situation. Company E sustained a number of casualties from mortar fire as well as rifles and machine guns.

At the time he received the message from the Company E runner at 12.00, Smoak was leading the rest of the battalion about 1,200 yards (1100 m) to the south of the junction of the two trails after being delayed by the late arrival of the forward observer team and difficulty in supplying his troops in their swampy assembly area. Smoak then led the rest of the battalion along the trail as swiftly as possible to support Company E. One platoon of Company F was left behind to provide security for the forward observer team’s wire team.

By 12.45, the battalion was 200 yards (185 m) to the rear of Company E, and Smoak now discovered that Company E was pinned down by heavy fire from Japanese ambush position to the south of the trail junction and taking casualties, and therefore required immediate reinforcement. Smoak ordered Company G, under the command of Captain William McDonough, to reinforce Company E, while Company H, under the command of Major Edward Clark, provided 81-mm (3.19-in) mortar support for the attack. Captain Robert Rapp’s Company F, less the platoon protecting the wire team, was ordered into reserve and to await further orders. The artillery forward observer team was ordered forward to Major Glenn Fissel’s Company E in order to make an estimate of the situation and call in artillery fire to prevent the Japanese from manoeuvring.

As the artillery forward observer team reached Company E’s lines, Fissel observed that the greatest volume of fire was coming from the eastern side of the trail from the area of the Piva river, and promptly requested an artillery concentration in that area. Receiving conflicting reports, Smoak needed to obtain more accurate information and thus moved his command post forward into the edge of the coconut grove through which the Numa Numa Trail ran. Fissel was able to make contact with Smoak and advise that Company E needed immediate support. After a quick reconnaissance, Smoak ordered Company F to pass through Company E, resume the attack, and allow Company E to withdraw, reorganise and take up a protective position on the battalion’s right flank. Company G, which had reached a position to the left of Company E, was ordered to hold where it was. Company F began its forward movement, and Company E, now with an opportunity to disengage itself, began a withdrawal and redeployment onto the right of the battalion’s position. Company F failed to make contact with either Company E or Company G.

During the withdrawal, Fissel was wounded. Unable to determine the exact locations on the various companies, Smoak sent several officers to determine the exact positions of his companies. Company F could not be found, and a large gap existed between the right flank of Company G and the left flank of Company E, which left the battalion in a dangerous situation. Smoak therefore ordered Company E to move forward, contact Company G, and establish a line to protect the battalion’s front and right flank. In the meantime, Company G was to extend its line to the right in order to link with Company E. By 16.30, Smoak had decided to dig in for the night, with his companies suffering fairly heavy casualties, Company F missing and communications with regimental headquarters and the artillery severed.

At 17.00, Company F’s gunnery sergeant arrived at the battalion command post, and reported that Company F had moved out as ordered from its reserve position to the line held by Company E, but had veered too far to the right and missed Company E. Company F proceeded onward and found itself in a position behind the Japanese lines. It was reported by the gunnery sergeant that Rapp had found it increasingly difficult to control his company as a result of casualties and the intermingling of platoons, which had become disorganised. The sergeant was ordered to return to Company F and guide it back to the battalion position. By 17.45, Company F had regained the battalion lines and had taken position on the perimeter which was set up for the night.

Communications were re-established at 18.30 and the 12^th Marines were ordered to set up positions on the northern, eastern and western sides of the 2/21^st Marines' perimeter. The 2^nd Marine Raider Battalion attached to the 21^st Marines, was directed to protect the supply line from the main line of resistance to the 2/21^st Marines. Ames instructed Smoak to send out patrols and prepare to attack the Japanese positions in the morning, with tank, artillery and aircraft support.

Throughout the night there was intermittent Japanese rifle fire, but no attempt to attack the marine positions. On the morning of 14 November, all the companies established outposts about 75 yards (70 m) in front of the perimeter, and started to send out patrols. At 09.05 air attacks were called in, and 18 Grumman TBF Avenger aircraft of the VMTB-143 squadron bombed and strafed the area marked with smoke by the artillery. Immediately after the air attack, Company E moved back into its original position in the line. Smoak then ordered an attack, with Company E on the left and Company G on the right while Companies F and H were held in reserve. The attack was to be a frontal assault supported by five M3 Stuart tanks of the 2^nd Platoon, Company B, 3^rd Tank Battalion.

The attack was scheduled for 11.00 but, as a result of the cutting of communications at 10.45, was delayed until communications had been re-established. Communications were restored at 11.15 and the attack then ordered for 11.55. The 2/12^th Marines, in direct support, were to provide a 20-minute artillery preparation followed by a rolling barrage.

After the preparatory barrage, the attack began at 11.55. The Japanese immediately reoccupied their positions and opened fire with rifles and machine guns. Several of the 3^rd Tank Battalion’s tanks became confused and fired into the marines, and accidentally ran over several men. Two tanks were damaged by Japanese anti-tank fire. There was a complete loss of control and wild shooting for about five minutes, Smoak moving forward in person to give orders to cease fire and halt the advance. The Japanese fire having stopped, Smoak directed all companies to hold in their current positions and launch patrols to a distance of 100 yards (90 m) to the north of the trail junction.

With the exception of the two damaged by Japanese anti-tank weapons, the tanks were ordered to return to an assembly position in reserve. At this time, it was discovered that the marines had overrun the Japanese positions, and that there were still some Japanese in their dug-outs. These were quickly reduced by riflemen with grenades. By 14.00, all Japanese resistance had been overcome, and the returning patrols reported no further contact. The advance was resumed at 14.15, and by 15.30 the objective had been occupied, whereupon a perimeter defence were organised for the forthcoming night.

It was estimated that the Japanese force was of company strength. The Japanese positions were very extensive and well organised, with many well-constructed machine gun positions and most dug-outs both deep and possessing good overhead cover. Although a careful count of the Japanese dead was not taken, it was estimated that at least 40 Japanese had been killed, and six machine guns were captured. The marines lost 20 men killed, including five officers, and 39 wounded.

This 'Battle of the Coconut Grove' paved the way on 15 November for an advance on all fronts, extending the perimeter of the Torokina lodgement to about 1,000 yards (915 m) on the left (west) flank and about 1,500 yards (1370 m) to the north in the centre to the inland 'Dog' defence line.

Artillery preparation was later recognised as a matter of primary importance against the Japanese system of defences, which was characterised by well dug-in, concealed and covered foxholes, equipped with a high proportion of automatic weapons, in turn covered by equally invisible riflemen in trees and foxholes. It had become evident that severe losses were inevitable for attacking infantry, regardless of the size of the force, unless the attack was preceded by artillery or mortar preparation, or by bombing or by all three.

By 15 November the USA had 33,861 men and 23,137 tons of supplies in the perimeter, and the 'Seabees' were working hard to build a road network within the perimeter. On 16 November the first such road was completed, and completion of the airfield inside the perimeter could begin in earnest. However, on 17 November Japanese aircraft finally inflicted a serious blow on the Americans, sinking destroyer transport McKean with heavy loss of life. Throughout the early part of November, the Japanese had carried out air raids against the US forces around Torokina, but by 17 November their losses were such that the 1^st Carrier Division, which had begun with 370 aircraft on 1 November, was withdrawn to Truk atoll in the Caroline islands group. Thus the US forces were the better able to undertake the steady expansion of their lodgement inland to a radius of between 5 and 6.25 miles (8 to 10 km), eventually establishing the Piva Fields base with two airstrips each containing a 5,000-ft (1525-m) runway from which they could subsequently launch their own attacks against Rabaul. Following this, the Japanese troops on Bougainville essentially became isolated.

On 21 November, the 3^rd Marine Raider Battalion spearheaded an advance along the East-West Trail from its junction with the Numa Numa Trail. The battalion crossed the crest of the trail to discover that it were looking down on the main Japanese positions to the east of the Piva river and astride the East-West Trail. The position also cut the Japanese supply line to the west. The marines raced to dig in before Japanese 90-mm (3.54-in) mortars could register, losing seven killed but holding the position. On the following day the 2^nd Marine Raider Battalion relieved 3^rd Marine Raider Battalion on the trail crest, and the rest of 3^rd Marine Division moved up to the 'Easy' line while the 37^th Division took over much of the north-western perimeter.

Meanwhile, on 20 November, the 2/3^rd Marines had spotted what came to be known as Cibik Ridge and, appreciating its significance as commanding high ground, promptly ordered a platoon to occupy the ridge. In the course of the morning morning, the marines on the ridge discovered that the Japanese had prepared the ridge for defence, but had pulled out for the night to avoid US artillery fire. When the Japanese attempted to move back into their positions, they were met by a hail of fire. The platoon commander called for mortars, and two 60-mm (2.36-in) mortars soon reached his position. Over the next two days, the Japanese engaged in mortar duels with the marines while attempting to storm the ridge, but their attacks were repulsed. The marines had secured a crucial position on high ground overlooking the battlefield.

The 2/3^rd Marines now undertook a reconnaissance in force to the east, only to discover that the Japanese had constructed a formidable defence line of about 20 pillboxes and that the 23^rd Regiment was massing for a counterattack. The marines disengaged with some difficulty and moved back behind the perimeter, only to be turned around and thrown back into the line to help repel the Japanese counterattack. On 21 November, the Japanese attempted a double envelopment of the positions of the 1/3^rd Marines, but were driven back by machine gun fire that inflicted heavy casualties.

The next two days were spent preparing for an attack on the 23^rd Regiment, whose strength was estimated at something between 1,200 and 1,500 men. The 37^th Division reached its final planned defensive line, the 'How' line, while the 3^rd Marines assembled their assault force and registered artillery on known Japanese positions. The marines had noted that the Japanese defences were located parallel with the high mountains to the north and were oriented to the south, leaving them vulnerable to an advance from the west.

The attack began on 24 November with the heaviest marine artillery bombardment so far in the Pacific War inasmuch as some 5,760 rounds were fired in 23 minutes. The Japanese had also registered their own artillery, however, and as the marines were moving up to the line of departure, a Japanese battery twice walked its fire up and down the assembling marines. The Japanese battery was spotted and silenced by marine counter-battery fire, but not before it had inflicted the heaviest Allied casualties of the campaign. However, the Japanese had also suffered heavily from the fire of the marine artillery, and the marines were able to move forward about 500 yards (460 m) before the Japanese rallied and counterattacked the marines' flank. The marines met the counterattack frontally and destroyed the Japanese flanking force. By the time the marines reached their objective, 1,150 yards (1050 m) ahead of their line of departure, all Japanese resistance to their front had ceased.

On 24 November, the 1/9^th Marines attacked to the north-east from Cibik Ridge, soon coming under heavy fire from a parallel ridge the Marines dubbed 'Grenade Hill'. Fighting was at such close quarters that the mortars on Cibik Ridge were unable to undertake support fire. On the morning of 26 November, however, the Marines found that the Japanese had abandoned Grenade Hill. The 23^rd Regiment had been shattered, with casualties of at least 1,196 dead. The marine casualties were 115 dead and wounded. There would be no further serious Japanese attacks on the perimeter until March 1944, though the Japanese would make frequent night air raids against the perimeter. Meanwhile the perimeter was wired and roadblocks established on all routes leading into it.

This was not quite the end of this initial Japanese counterattack, which in fact came to a temporary conclusion with a US reverse in the Koiari raid of 28/29 November. The raid involved a landing by a battalion-sized force of US marines to harass Japanese troops, and was a failure for the Americans as they were counter-attacked by a Japanese force somewhat larger than had been expected, and as a result the marines were withdrawn from the beach-head without achieving any of their objectives.

The origins of the raid can be found in the desire of Geiger to increase the perimeter of the lodgement which had been established around Cape Torokina to include a ridge of hills 2,000 yards (1830 m) away. Geiger ordered an advance to the east in the direction of the Torokina river to establish a 'Fox' inland defence line. The advance was to start on 26 November and, in order to shield the general advance from any surprise Japanese attack on its right flank, Geiger planned a raid at Koiari, 10 miles (16 km) to the south-east along the coast from Cape Torokina: this raid was to detect Japanese troop movements, destroy Japanese supply dumps and disrupt the Japanese line of communications. The raiders were to harass Japanese units as far inland as the East-West Trail, but at the same time to avoid any decisive engagement with major Japanese forces.

Major Richard Fagan’s 1/1^st Marine Parachute Regiment, which had just arrived from the island of Vella Lavella, was assigned as the main force, with Company M of the 3^rd Marine Raider Battalion and an artillery forward observer team from the 12^th Marines attached to create an overall strength of 614 men. As a result of delays, however, the raid was postponed to 29 November. Covering fire for the landing, which was to have been provided by destroyers, was now no longer available. A preliminary reconnaissance landing was undertaken in the dark hours of 27/28 November, and reported no Japanese activity in the area. A boat reconnoitred the planned beach landing site and also reported no Japanese activity.

The 1^st Marine Parachute Battalion was transported by medium landing craft and vehicle/personnel landing craft of the US Navy, which landed the attack force at Koiari at 04.00 on 29 November in an area beside a Japanese supply dump. The marines overran the supply dump and quickly dug in 200 yards (185 m) inland amid heavy mortar, machine gun and rifle fire from the Japanese defenders. Company M of the 3^rd Marine Raider Battalion and the HQ Company were meanwhile landed 880 yards (805 m) farther to the south.

The Japanese now committed a number of infantry charges which caused significant casualties amongst the US forces. Close support fire was provided by 155-mm (6.1-in) guns of the 3^rd Marine Defence Battalion firing from their positions in the Cape Torokina area under the control of the forward observation teams. A captured 37-mm gun was also used against the Japanese.

Experiencing resistance which was altogether greater than had been expected, the US commanders quickly appreciated that the raiding force was outnumbered. With an estimated 1,200 Japanese in the immediate vicinity, the US raiding force faced annihilation. The second landing party, which had been landed farther to the south, linked with the main party at 09.30 after suffering 13 casualties as it moved to the north. Realising the futility of the situation and believing that he no longer possessed any freedom of action, Fagan sent a request by radio for evacuation. Geiger concurred and ordered the marines' withdrawal, although a communications failure resulted in this message not reaching Fagan.

Two attempts were made to extract the marines by landing craft, but these both failed as a result of the weight of the Japanese artillery fire. The marines now had their backs to the sea and were running short of ammunition, so an infantry landing craft gunboat and the destroyers Fullam, Lansdowne and Lardner, which had been summoned from convoy escort duties, closed in to the beach at 18.00, and in combination with shore-based artillery and close air support provided a barrage which made it possible for the rescue craft to remove the raiding group from the beach: the last boat left the area at 20.40 under cover of darkness.

The raid achieved none of its objectives, and was thus a failure costing 15 men killed or died of wounds, 99 wounded and seven missing. It was estimated that the Japanese had lost between 145 and 290 men. Again the lack of preliminary naval and artillery bombardment had proved to be important in the outcome of the engagement. Despite this failure, however, US plans to expand the perimeter around Cape Torokina went ahead with units of the 3^rd Marine Division advancing toward the Torokina river throughout early December, as part of a plan to occupy the high ground west of this same watercourse. This advance would see the marines take part in a series of engagements around 'Hellzapoppin Ridge' and Hill 600A in the middle of December.

This first, albeit limited, Japanese counterattack had thus failed, and by the beginning of 1944 the lodgement contained three airfields, a naval base and a vast supply dump defended by Major General John R. Hodge’s Americal Division (132^nd, 164^th and 182^nd Infantry) and Beightler’s 37^th Division (129^th, 145^th and 148^th Infantry) which, since 15 December 1943 had been under the command of Major General Oscar W. Griswold’s XIV Corps.

By the beginning of 1944, with their beach-head perimeter secured and airstrips for both bombers and fighters completed and in service, the US forces deemed the initial phase of the Bougainville campaign to be over. The Americal Division had taken over the eastern part of the perimeter from the 3^rd Marine Division, and the VMF-216 and VMF-70 fighter squadrons were operating out of the airfield. Allied intelligence reported that the Japanese in the northern part of Bougainville were digging in around the Buka and Bonis airfields, and were unlikely to move against the US perimeter. It was the opinion of US intelligence that the primary Japanese threat was likely to be from the southern part of Bougainville, where the greater part of the 6^th Division, numbering some 11,000 men, could start to advance against the perimeter using trails which were well concealed from aerial reconnaissance by the dense jungle canopy.

The US forces carefully prepared the perimeter for defence any Japanese counter-offensive, building an extensive complex of pillboxes and other fortifications to provide interlocking fields of fire, laying barbed wire, constructing booby traps, clearing extensive clear fields of fire, and installing searchlights to offset the Japanese predilection for night assaults. Meanwhile the US forces undertook a programme of aggressive patrolling outside their perimeter, where Japanese activity was clearly increasing by February 1944.

One particularly celebrated patrol was that by the 1/Fiji Regiment, which moved out of the perimeter on 28 December 1943 and along the Numa Numa Trail to Ibu, from which the patrol was to fan out and report on activities on Bougainville’s east coast in an extended undertaking made logistically feasible by the use of air drops to keep the Fijians supplied. The patrol reached Ibu on 2 January, hacked out an airstrip for Piper L-4 Grasshopper observation and liaison lightplanes, engaged in a number of firefights with Japanese patrols, and were attacked on 14 February by a large Japanese force. The Fijian patrol fought an expert rearguard action as it steadily back to the US perimeter, which it reached on 19 February after claiming 120 Japanese killed in exchange for one Fijian slightly wounded.

On 27 February the US forces occupied the two small islands of the Magine group, just off the mouth of the Piva river, in order to deny their use to the Japanese for observation of US beach activity and conversely to serve as a US observation post from which to watch Japanese activities in the area to the east of the Torokina river.

Imamura, commanding the 8^th Area Army in Rabaul, was convinced that the US forces intended to seize the whole of Bougainville and, after a flying visit to Hyakutake’s headquarters on 21 January, ordered that the Empress Augusta Bay lodgement should be destroyed as soon as possible by Kanda’s 6^th Division (13^th, 23^rd and 45^th Regiments reinforced by elements of the 53^rd and 81^st Regiments of Sakai’s 17^th Division), as well as the 6^th Field Artillery Regiment and 10^th Mountain Artillery Regiment, with the air support of elements of Lieutenant General Kumaichi Teramoto’s 4^th Air Army.

The reinforced 6^th Division and the artillery regiments immediately began a major training effort as roads were built in the north-west and supplies were built up. The road-building endeavour was subjected to heavy US air attacks, and most Japanese barges were destroyed, forcing the Japanese to move everything by land. At the time of the 'Ta' (iii) offensive on 8 March, the XIV Corps' perimeter had a beach frontage of 11,000 yards (10060 m) and a depth of 8,000 yards (7315 m) inland.

By this time Hyakutake had finally come to see the reality of the Japanese situation on Bougainville, and despatched his main ground strength, most especially the 6^th Division, to force its way through the jungle to attack the Cape Torokina perimeter. The Japanese assault forces were not able to finish assembling outside the US perimeter and launch their 'Ta' (iii) offensive until 7/8 March 1944. By then the 37^th Division was well dug in and fully alert, as a result of prisoner interrogations, of the forthcoming attack. (The 3^rd Marine Division had left the island in January 1944, returning to Guadalcanal for rest and rehabilitation pending its next operational deployment, which was to the island of Guam in the Mariana islands group during July 1944 within the 'Forager' operation.)

As was so often the case during the Pacific campaign, the Japanese plan was complex. This was unfortunate for the Japanese, for their local superiority of numbers, ability to move under jungle cover and possession of high ground overlooking the US perimeter offered the possibility that a concentrated attack at the right point could have posed a major threat to the US position. Hyakutake assumed that his forces were opposed by only one division rather than the two which were now holding the perimeter, which contained 62,000 US troops, and was therefore so confident that the combat troops he committed to the attack could crack the perimeter that he had already planned the US surrender ceremony on 17 March. As a result, the Japanese assault forces were issued with rations for a mere two weeks.

Overall command of the offensive was allocated to Kanda, whose 6^th Division had been reinforced, as noted above, by two battalions of the 53^rd Regiment and part of the 81^st Regiment. Kanda divided his force into three columns. The first, under the command of Iwasa, the 6^th Division's infantry group commander, comprised the 23^rd Regiment, one battalion of the 13^th Regiment and supporting elements totalling some 4,150 men. Its objective was Hill 700 on the right flank of the 37^th Division, and after taking this the column was to drive down onto the Piva airfields. The second column was built around the 45^th Regiment, totalling some 4,300 men under the regimental commander, Colonel Isao Magata, and was to strike across the low ground to the west of Hill 700 and join the assault on the airfields. The third column, comprising two battalions of the 13^th Regiment and one company of engineers, totalling 1,350 men under the command of Colonel Toyhorel Muda, was to take Hill 260 and Hill 309, and then move forward to take Hill 608 and secure Iwasa’s flank. The Japanese columns also included a number of other elements raising their overall strength to 11,700 assault troops, and these were backed by 3,700 support troops.

The Japanese had used most of January and February to build an improvised road from Mosigetta to their assembly areas in the hills lying to the north of Torokina. Originally scheduled for 6 March, the offensive was postponed to 8 March as a result of delays in concentrating the troops earmarked for the operation. 'Ta' (iii) finally started with an artillery duel in which the Japanese, firing their heaviest bombardment of the entire Solomon islands campaign, forced the evacuation of the aircraft on the Piva airstrips to Guadalcanal but otherwise did little damage, while the US artillery was supplemented by 56 Douglas SBD Dauntless dive-bombers and 36 TBF level bombers which struck the Japanese concentration area on Hill 1111. The US forces were now on full alert at a moment when the Japanese infantry had not even arrived in force immediately outside the US perimeter.

Hill 700 consisted of two high points separated by a saddle, its approaches from almost every direction having a slope of 65 to 70°. Beightler had not anticipated an attack on so commanding a position, so the hill was held by just two infantry companies and a heavy weapons company of the 2/145^th Infantry, though these were notably well entrenched. Shortly after midnight on 9 March, in a heavy fall of rain, the 2/23^rd Regiment attacked the hill and the area to its west, but was driven back. Two hours later, Iwasa launched his main attack, committing the 2/23^rd Regiment and 3/23^rd Regiment against the saddle. The 3/23^rd Regiment, constituting the Japanese second wave, was destroyed by the US artillery, but the 2/23^rd Regiment in the lead managed to reach the US line, blow apart the barbed wire barriers with bangalore torpedoes, destroy a pillbox and secure a lodgement. By dawn the Japanese had achieved a penetration 70 yards (64 m) wide and 50 yards (46 m) deep, and continued to expand this until 12.00, in the process taking seven pillboxes and bringing up heavy weapons to interdict the US supply route to the south of the hill. Beightler committed the 1/145^th Infantry to help contain the Japanese, but it took another three days and massive artillery support for the Americans to drive the Japanese off the hill and restore the position. By then Beightler had been forced to commit a second battalion, the 2/148^th Infantry, and the commander of the 145^th Infantry had succumbed to combat fatigue. The battle had cost the Americans 78 men killed, while at least 309 Japanese dead were counted in the area.

Meanwhile, at dawn on 10 March, the Muda column had struck at Hill 260, which accommodated a small observation post of the 2/182^th Infantry well outside the main US perimeter. Although the hill had a valuable observation platform built high in a banyan tree, the staff of the Americal Division were surprised when Griswold ordered the position to be held. By then the Japanese had already overrun most of the US positions, which were concentrated on the southern peak of the hill ('South Knob'). The Americans still occupied the hill’s northern peak ('North Knob'), but were unable to drive back the Japanese, although US artillery destroyed the banyan tree together with its observation platform. The Americans abandoned their effort to retake the 'South Knob' on 20 March, leaving the position to be battered by artillery and contained by patrols, and on 28 March a patrol discovered that the Japanese had withdrawn. The struggle had cost the Americal Division 98 men killed and 581 wounded, while some 560 Japanese dead were counted around the position.

Magata’s column was slow to attack, which was unfortunate for the Japanese, since the terrain in its sector was flat and therefore favoured the attackers. The Americans had captured documents revealing Magata’s plan, and the defending 129^th Infantry had spent the previous two months building a formidable defence line, with mutually supporting pillboxes protected by double-apron barbed wire barriers and minefields, and equipped with large numbers of machine guns, 75-mm (2.95-in) pack howitzers, and 37-mm anti-tank guns supplied with canister anti-personnel ammunition. At 16.00 on 11 March, the commander of the 129^th Infantry ordered his outposts back to the line of pillboxes, and the divisional artillery pounded the area in front of the 2/129^th Infantry’s position for 10 minutes.

After dusk there broke out a firefight in which the Americans were careful not to reveal their pillbox positions but were unable to prevent Japanese infiltrators from cutting gaps in the barbed wire. Magata attacked at dawn with two battalions, and by sheer weight of numbers captured seven pillboxes. The 1/129^th Infantry was sent into the line from reserve and recaptured two pillboxes, and in the course of the evening which followed, searchlights illuminated the low cloud cover while the US troops automatic fire into the Japanese positions. On the following day, M4 Sherman medium tanks of the 754^th Tank Battalion were committed to the fight, and by 14 March the US position had been restored. Another attack by Magata’s force in the hours just before dawn on 15 March took a single pillbox before being contained and driven back with the aid of tanks and an attack by 36 aircraft. A third Japanese attack on 16 March made little progress. This final attack cost the Japanese 194 men killed and one man taken prisoner in exchange for US casualties of two men killed and 63 wounded.

Kanda now ordered a withdrawal to regroup, assembling what was left of his force to make a final attempt to break through the line of the 129^th Infantry. During the following five days, the Americans rebuilt damaged positions and buried the Japanese dead.

Meanwhile Allied codebreakers had intercepted and decoded a message from Hyakutake to Imperial General Headquarters to advise of his plan to attack once again on 23 March. The intercepted information was rushed to Griswold, who was already dealing with a preliminary assault which had seized a low ridge very close to the battalion command post of the 2/129^th Infantry. The Japanese penetration was driven back by tanks and infantry on 24 March, and shortly after 12.00 the Japanese assembly areas opposite the 129^th Infantry were hit by the heaviest artillery barrage of the Pacific campaign up to this time, a counter-preparation concentration of 14,882 shells delivered by seven battalions of heavy and medium artillery as well as the mortars of the 37^th Division. Two days later the Japanese began to retreat to their base area at Buin in the extreme south of Bougainville.

The 37^th and Americal Divisions, backed by a tank battalion as well as powerful air, artillery and naval support (Captain Wallis F. Petersen’s Destroyer Squadron 22 with Pringle, Conway, Sigourney, Eaton, Renshaw and Saufley), had held firmly.

The casualties in the 'Battle of the Perimeter' included 263 Americans killed and about 5,500 Japanese dead, the latter increased to 5,470 men killed and 7,000 wounded as a result of US air and naval attacks in the rest of Bougainville. Part of the reason for the crushing defeat suffered by the Japanese was the poor condition of their troops. US medical personnel who examined Japanese prisoners of war and Japanese dead arrived at the conclusion that nine out of every 10 Japanese soldiers were already suffering from malnutrition, malaria, beri-beri or one of several other debilitating diseases. Kanda had adopted the same concept as Lieutenant General Renya Mutaguchi, who in the approach to the Battles of Imphal and Kohima had disastrously promised the men of his 15^th Army that they would feast on the Allied supply dumps once they had broken through to them.

The 'Battle of the Perimeter' had effectively broken the fighting capability of the 17^th Army, despite the availability of another 32,000 men.

Following the end of this battle, Griswold came to the conclusion that the Japanese on Bougainville could no longer have any influence on the outcome of the war, and therefore instructed his divisional commanders to undertake nothing more than aggressive patrolling as the isolated 17^th Army was left almost literally to rot without food, medicine, clothing and other supplies. Elements of Major General Raymond G. Lehman’s 93^rd Division, which was a segregated African-American formation, began to arrive at this time, and while some of its men were used in small-scale patrol undertakings, the division’s manpower was used primarily for stevedore work. In April Major General Robert B. McClure assumed command of the Americal Division when its previous commander, Hodge, was elevated to command of the XXIV Corps. The Japanese were completely cut off from resupply: by April 1944 their rice ration had been cut to 8.8 oz (250 gr) per day, and in September the rice supply was exhausted, most of Hyakutake’s men then being put to work on garden plots. Allied pilots took to dropping napalm on the Japanese gardens, and Japanese morale plummeted to the point where desertion was common and there was real danger of mutiny.

With Japanese morale at its nadir, and the Americans disinclined to stir up trouble pointlessly, what was in effect an unspoken truce settled over the perimeter. The Japanese had dug in on the south-eastern and north-western ends of the island while the US forces, rather than undertake major offensive operations, restricted themselves to aggressive patrols and limited-objective attacks to expand the outpost line. Most contacts were made north-east of the perimeter. Thus there emerged what was in effect a no man’s land in which the Japanese and US forces generally keeping their distance, although patrols did engage each other. Besides perimeter security and patrols, the two US divisions undertook combat training to include amphibious exercises to prepare for future operations.

From October 1944, the XIV Corps was steadily withdrawn for commitment in the campaign for the Philippine islands group, and responsibility for the Allied perimeter was taken over by Lieutenant General Stanley G. Savige’s 30,000-man Australian II Corps. This comprised Major General William Bridgeford’s Australian 3^rd Division (Australian 7^th, 15^th and 29^th Brigades) and the independent 11^th and 23^rd Brigades. Savige assumed command of the perimeter on 22 November, and by 12 December all US units had been withdrawn from the front line.

General Sir Thomas Blamey, commander-in-chief of the Australian Military Force and commander-in-chief of the Allied Land Forces South-West Pacific, and Savige decided to undertake active operations against the Japanese rather than remain idle on the perimeter. The reasons for this controversial decision remain obscure. However, MacArthur had all but shut the Australian army out of the counter-offensive against Japan, relegating Australian units to secondary operations and garrison duty, and Blamey may have felt that the reputation of the Australian army as a high-quality fighting force was at stake.

A genuinely major effort by the Australians to eliminate the Japanese on Bougainville island could not be undertaken for lack of the required air and naval support. Between January and June 1945 the Australians therefore concentrated their combat patrols and sweeps in three areas: the Numa Numa Trail and the north-eastern coastal area, the north-western coast to the Bonis peninsula, and the coast to the south-east of Empress Augusta Bay though not as far as the heavily defended Buin area on the south-eastern end. Savige knew that the 6^th Division, which was the core of the Japanese garrison, was located round Buin in the south of the island, and concentrated most of the 3^rd Division on his southern flank. Brigadier Arthur W. Potts’s 23^rd Brigade was ordered to patrol aggressively to find the Japanese defences, and by the time Savige ordered a general offensive on 23 December, the Australians had already made several strong probes into Japanese-held territory. Savige knew his force was probably outnumbered, but also that the Japanese troops' poor morale and dire physical condition offset this advantage: the Japanese were in fact dying of illness and starvation at the rate of 3,000 men per month.

The offensive opened with an attack on Pearl Ridge by the 25^th Battalion on 30 December, and this succeeded in driving 500 defenders off a strong position in just two days. From here the Australians could observe both coasts of the island. The Australians then installed a 3,000-ft (915-m) cable to the ridge and used it to pull a bulldozer to the top to begin the construction of a road. The ridge then became a major patrol base commanding the central part of the island.

On 31 December, Brigadier John R. Stevenson’s 11^th Brigade moved off toward Soraken Point on the island’s north-west coast, the brigade’s final objective being to force the Japanese in the northern part of the island into the Bonis peninsula, where they would then be destroyed. The brigade advanced rapidly until 19 January, when advance patrols discovered that the Japanese were dug in on Tsimba Ridge. Savige was reluctant to commit his armour against this position, and it was not until 6 February that the Japanese were driven off this feature. On 26 March a combined ground attack and amphibious landing forced the Japanese out of Soraken Point and into the Bonis peninsula.

In February 1945 Hyakutake, commander of the 17^th Army, suffered a stroke and was replaced by Kanda, who was succeeded in command of the 6^th Division by Lieutenant General Tsutomi Akinaga, also known as Tsutomu Akinaga, who had been chief-of-staff of the 17^th Army up to this time and was now succeeded by Magata, previously commander of the 45^th Regiment.

The 1,000 Japanese at Kieta on the central part of the island’s east coast were contained by guerrilla forces, which were led by Australian coastwatchers, came quickly to control much of central Bougainville and claimed to have killed more than 2,000 Japanese by the end of the war.

The main effort in the south of Bougainville started on 28 December, and was led by Brigadier Raymond F. Monaghan’s 29^th Brigade, which met its first major resistance on the Hupai river during 10 January. Here the Australians were forced to bring forward anti-tank guns in order to clear a line of pillboxes, and then they advanced swiftly into more open terrain where, on 11 February, they took Mosigetta. Anticipating more serious resistance in the area farther to the south, Savige committed his armour on 17 March, and on 19 March the 25^th Battalion cleared a system of pillboxes on its line of advance. After the receipt of an intelligence warning that Kanda was about to launch a major counterattack, the Australians dug in around a terrain feature dubbed Slater’s Knoll. An initial pair of Japanese probing attacks on 27 March was followed by the main assault three days later, and this struck a single Australian company in the area to the south of Slater’s Knoll. The Australians beat off four bayonet charges. but at the end of the fighting only 16 Australians in the position remained unwounded, and the line was pulled back to Slater’s Knoll. The Japanese counterattack had exhausted itself on 5 April, when waves of Japanese charged into massed automatic weapons fire and artillery barrages. Out of a force of 2,400 of Kanda’s fittest troops, at least 620 were killed and another 1,000 wounded. Thereafter Kanda reverted to a purely defensive strategy and began moving troops from the Shortland islands group to reinforce his surviving strength in the Buin area.

Meanwhile Stevenson’s 11^th Brigade in the north was nearing exhaustion. An attempt to flank the Japanese lines with an amphibious landing on 8 June nearly ended in disaster, and the Australians made no further attempt to clear the Bonis peninsula during the remaining weeks of the war.

Savige took another two weeks to regroup his forces in the south, then resumed a slow advance towards Buin, supported by powerful tactical air attacks by Vought Corsair fighter-bombers of the Royal New Zealand Air Force. By 3 July the Australians were ready to launch their final assault toward Buin, but drenching rain repeatedly forced Savige to postpone the offensive, which was then cancelled on 11 August when news arrived of the imminent Japanese capitulation.

The Australians had suffered casualties of 516 men killed and 1,572 wounded without driving the Japanese out of Buin. The Japanese in turn surrendered just 21,090 soldiers and sailors out of a force that had started with perhaps 65,000 men.