Operation Battle of Slater's Knoll

The 'Battle of Slater’s Knoll' was a battle between Japanese and Australian forces on Bougainville island in the last part of the Bougainville campaign in the Solomon islands group (28 March/6 April 2945)[/colour].

The battle occurred as a force of about 3,300 Japanese soldiers of Lieutenant General Masatane Kanda’s (from 1 April Lieutenant General Tsutomu Akinaga’s) 6th Division, including artillery and other supporting elements, launched a counterattack against the main Australian offensive which had been pushing to the south in the direction of Buin, concentrating their attacks on Slater’s Knoll near the Puriata river. The Australian troops belonged to the 7th Brigade, with the 25th Battalion the most heavily engaged of its units, although the 9th Battalion and the 61st Battalion were also involved in the fighting.

Against Japanese tactics that included massed attacks, the Australians made extensive and effective use of armour and artillery, which proved decisive. Starting late in March, after the Australian advance had been halted by wet weather, over the course of several days the Japanese launched several probing raids followed by heavy attacks. The final assault on the knoll came on the night of 4/5 April when 129 men of the 25th Battalion’s B Company repulsed an attack by a force of about 1,100 Japanese, in the process killing 292 of them. This proved to be the 7th Brigade’s final involvement in the campaign as it was relieved by the 15th Brigade shortly after this. Overall, 620 Japanese were killed in the battle with another 1,000 estimated to have been wounded, while the Australians suffered 189 casualties.

At the beginning of 1945, the south-eastern area of Bougainville was held by the 6th Division, which comprised the 13th Regiment and 23rd Regiment, with only 600 and 700 men respectively, as well as the 6th Field Artillery Regiment, the 6th Engineer Regiment, the 6th Transport Regiment and the 4th Field Heavy Artillery Regiment for a total of 3,300 men. The Australian estimate of this force’s strength was between 2,400 and 2,700 men.

During previous November, Lieutenant General S. G. Savige’s Australian II Corps had taken over responsibility of the island from Major General Oscar W. Griswold’s US XIV Corps, which was redeployed for the Philippine islands campaign. At that time the Allies incorrectly believed that the Japanese forces on the island numbered around 17,500 men: Allied estimates varied greatly and it was only after the war that it was found that there had been more than 40,000 Japanese on the island at the time. While these forces were under-strength, they were still considered to be capable of carrying out effective combat operations. In order to counter this, it was decided that the Australian II Corps, comprising militia and Australian Imperial Force troops of the 3rd Division and the 11th and 23rd Brigades, would go on the offensive and a three-pronged campaign was planned in the northern, central and southern sectors of Bougainville island. Also attached to the Australian corps were the 2nd and 4th Field Regiments, one company of the 1st New Guinea Battalion, the 2/8th Commando Squadron, and one squadron of Matilda II infantry tanks from 2/4th Armoured Regiment.

As a result, the Australian campaign on the island developed into three separate drives: in the north, it was planned that Japanese forces would be forced into and then contained in the narrow Bonis peninsula; in the centre, the seizure of Pearl Ridge would give the Australians control of the east/west lines of communication and protection against further counterattacks, while also opening the way for a drive to the east coast; in the south would be the main campaign, in which the bulk of the Japanese forces were concentrated. On 21 January, therefore, Savige directed Major General W. Bridgeford’s 3rd Division to take 'swift and vigorous action' to destroy Japanese forces in southern Bougainville. The division’s immediate task was to advance southward to the Puriata river, and send patrols across it. Savige considered that the Japanese division was 'weak and off balance' as a result of its casualties in battle and losses to disease as well as a shortage of supplies. Brigadier R. Monaghan’s 29th Brigade had opened the campaign in the south in the middle of December, crossing the Jaba river, before conducting a series of landings by barge along the coast of Empress Augusta Bay, outflanking the Tuju, Tavera, Adele and Hupai rivers, in what became dubbed the 'Battle of the Swamps'. The fighting during this time resulted in 240 Japanese being killed, while the Australians suffered 148 casualties. Monaghan was also relieved of his command and replaced by Brigadier N. Simpson.

Late in January, Brigadier J. Field’s 7th Brigade, which had been patrolling the Jaba river to protect the 29th Brigade’s rear while this pushed to the south, was subsequently tasked to 'take Mosigetta, clear the enemy from the Kupon-Nigitan-Sisiruai area, and patrol along the Puriata'. Each of the brigade’s three infantry battalions was marginally below full strength at the time, with strengths of between 600 and 700 men at the time of the bttle’s start.

Late in January, a company of Lieutenant Colonel J. McKinna’s 25th Battalion subsequently carried out an amphibious landing to the south of Motupena Point on the Solomon Sea coast, and cleared the area toward Matsunkei. On 2 February, the battalion carried out another landing, this time at Toko, and advanced to the north-east from there toward Batara, and thence along the western bank of the Puriata river toward the Buin road. Meanwhile, farther to the east, the 9th Battalion was moved by truck from Torokina to the Jaba river and then moved by barge along the Empress Augusta Bay coast, landing at Mawaraka, from where it advanced toward Mosigetta, and the 61st Battalion also proceeded toward that area after departing from the junction of the Jaba and Pagana rivers, while the 2/8th Commando Squadron provided flank security farther to the east. Elements of the 1st New Guinea Battalion also conducted a landing on the coast, on the 25th Battalion’s left flank, to the north of Toko, and advanced to Makaku and thence toward Mosigetta.

On 4 March, one company of the 25th Battalion crossed the Puriata river to the south, at the point where the Buin road crossed, on a north-facing bend, later known as Galvin’s Crossing. On the following day, the Japanese were forced off a small knoll close to the river and the road, about 1,100 yards (1005 m) south of the crossing. On 6 March, Japanese shelling of the knoll wounded Private John Slater, who held his post until relieved and was the only Australian casualty of this period; the knoll was named after him. Throughout March, the 25th Battalion continued to expand its perimeter, to the north and south along the Buin road, while the 9th Battalion moved to the west from Mosigetta to link with it, and the 61st Battalion skirted to the east around Makapeka. By a time late in March, though, heavy rain had brought the Australian advance to a temporary halt as the main road became impassable as a result of thick mud.

Although the Japanese sporadically shelled the knoll during March, there were no more Australian casualties. The 25th Battalion, which had established its headquarters and main defensive position around the knoll, in front of the Puriata river, despatched patrols every day, and on 9 March one of these reported having killed 10 Japanese soldiers without loss to itself. The Japanese also began making attacks at down, when light machine gunners approached by stealth, fired on the Australian positions and then retired to their own lines, 250 yards (230 m) away. As the advance was halted, the Australians began to expect a major attack in the area of the knoll.

The Japanese launched a number of attacks on the Australians between 15 and 17 March. Under fire from three sides and in danger of being enveloped, on 19 March the Australians launched their own attack along the Buin road: although they had some success in clearing the forward Japanese positions, the Australians then encountered a system of pillboxes and trenches around the Hatai road junction. McKinna, the 25th Battalion’s commanding officer, ordered the two platoons led by Lieutenant Dick Jefferies to attack the pillboxes, supported by a section of machine guns and several PIAT anti-tank weapons. There followed a two-hour fire-fight and a bayonet charge. This attack was ultimately beaten back by the defenders with the loss by the Australians of eight men killed and 14 wounded. A second attack was launched on 22 March. Supported by heavy artillery, mortars and machine guns, it proved successful, largely after the individual efforts of Corporal Reg Rattey, who single-handedly silenced several bunkers.

As the Australians began to prepare for the next stage of the offensive, important intelligence confirmed that the Japanese were planning a large-scale counterattack in the area. Believing that the Australian forces in the area surrounding the knoll were only 400 men and observing that the Australians had not had time to reorganise, Kanda, now commander of the 17th Army, ordered Akinaga, his successor as commander of the 6th Division, to delay the Australian advance to the south. Akinaga assessed that a counterattack against the Australian forces at this point with the entire weight of his division might be successful, so he gave the order for his division to begin an attack across the Puriata river. Akinaga planned to commit 2,000 men to the assault, while 1,300 would be held back to act as ammunition carriers.

It is possible that no detailed orders were produced for the attack and deliberate planning was hastily completed with little or no co-ordination between the attacking regiments. In addition, to increase the size of the infantry force, troops from the field artillery regiment were quickly re-roled. In the meantime, the Australians on Slater’s Knoll, having been alerted to the possibility of a large-scale attack, began adjusting their defences. The soldiers cleared fields of fire and set out precise engagement areas to the north, north-west and south of the knoll to allow maximum effectiveness of their supporting medium and light machine guns. Barbed wire was laid out, and booby traps set.

The main attack was planned for 1 April, but as the various Japanese units moved into position, a number of minor probes were made by elements of the 6th Engineer Regiment into the Australian rear areas and along their line of communications. The first raid was made on the night of 27/28 March, when a force of about 100 Japanese exploited a gap between the 25th Battalion’s main defensive position and its 'B' Echelon, which included its transport and logistics elements and was protected by a company of the 61st Battalion, fixed bayonets and then assaulted the rear of the Australian perimeter. The latter had been alerted to the approach of the Japanese after the communication line between the two positions had been cut, and when the attack began the men had already been roused and the weapons pits fully manned. As a result, the attack failed and was turned back. A number of Japanese survivors managed to dig themselves into the area near the perimeter and later fired upon the Australian clearing patrol that was despatched during the following morning, and also sniped at the company defending the 'B' Echelon’s position. After this, the Australians counted 19 Japanese dead around their position, while they also managed to capture one of the wounded. The 25th Battalion had lost three men killed and seven wounded.

Another probe followed early in the morning of 28 March on the positions of the 5th Field Battery, whose eight guns were located to the east of Toko, on the western bank of the Puriata river, from where they could provide defensive fire in support of the 25th Battalion’s main positions on the knoll. Shortly before dawn one of the booby traps set by the Australians around their perimeter to provide early warning was set off and one of the sentries opened fire upon what he believed to be a Japanese probe. A short time later, a small section of Japanese stumbled into one of the forward positions and fire was exchanged. Later, as a team was sent out to disarm the remaining booby traps, this team was fired upon and sporadic fighting continued until about 12.00.

At about 23.00 during the evening of the same day, the 9th Battalion’s rear echelon at Barara, along the Toko-Mosigetta-Buin road, came under attack. Led by the battalion second in command, and possessing only four light machine guns for direct fire support, the men of the rear echelon were nonetheless able to hold off the initial attack. After this, sporadic fighting continued into the early morning, and at 04.45 on 29 March a force of about 100 Japanese attacked the position with the support of sustained machine gun and rifle fire. Rushing the Australian position with bayonets, the Japanese were beaten off and fell back, leaving 23 dead but taking their wounded with them. Four Australians were wounded in this encounter.

Throughout 29 March, the Japanese continued the tactic of using small-scale attacks to test the Australian defences and isolate the 25th Battalion’s forward companies from its headquarters and rear. During the morning and into the afternoon, one of the Australian battalion’s companies was probed, while the headquarters also came under attack. The 61st Battalion’s company protecting the 25th Battalion’s line of communications also came under attack from the 70 Japanese believed to be entrenched between the 'B' Echelon and the 25th Battalion’s main positions, suffering two killed and two wounded. It later became clear, following the capture and interrogation of a Japanese sergeant, that the Japanese were preparing for a major attack on Slater’s Knoll.

These probes intensified on 30 March, when one of the 25th Battalion’s companies came under strong attack. The Australian position had been weakened by having about one-third of its strength out on patrol at the time of the attack. Unable to return to the perimeter while the company was under fire, the patrol was forced to link with one of the other companies farther to the north. Meanwhile, the company was attacked four times throughout the day. The first three attacks were repulsed. The last attack came at 13.00, preceded by a heavy mortar bombardment. Some 12 Japanese were killed in the attack, while one Australian was killed and two were reported missing in action. Nevertheless, the Australian position had been made untenable and the company was compelled to withdraw and link with the company to the north. In doing so, it left behind three mortars and a quantity of ammunition, which the Japanese later brought into action against the Australians.

Communications between the brigade and battalion headquarters and the forward company’s positions broke down at this stage and runners had to be used to pass a message asking for artillery support. Meanwhile, the Australian supply situation also became desperate as they had eaten all their rations and had run out of water. The latter situation was solved by the digging of a well within the perimeter, but it became clear that reinforcements were required. Bridgeford, the Australian divisional commander, subsequently placed at the 7th Brigade commander’s disposal, a squadron of the 2/4th Armoured Regiment’s Matilda II infantry tanks, under the command of Major Kenneth Arnott, and during the course of the day these tanks were brought toward the 25th Battalion’s position from Toko, where they had been put ashore from landing craft. Engineers were used to bridge a number of creeks and streams, while bulldozers were also employed to improve the terrain or to tow the tanks when they were unable to move under their own power.

This was the first time tanks had been used in the campaign, and the going was slow. By 19.00 on 30 March the tanks had reached the 'B' Echelon position. Early on 31 March the tanks moved up to the battalion headquarters on Slater’s Knoll, where they were escorted by a platoon from the headquarters company and moved out again, once again using bulldozers to improve the ground in front of them. Just as they did, the Japanese launched an attack on one of the isolated companies. When they were 400 yards (365 m) from the forward positions, the infantry deployed to the left and right of the tanks and began their advance to contact, arriving just in time to help defeat the Japanese attack.

As the forward companies began to move back towards the main defensive position, Jeeps were sent up to collect the wounded while McKinna led a force of two platoons and two tanks out to the perimeter at the Hatai junction. There they were able to reclaim and destroy the mortars which had earlier been abandoned, but the Jeeps carrying the wounded ran into a Japanese ambush in which five Australians were killed. McKinna quickly organised a counterattack, taking a force up the road along with three tanks and subsequently attacking the ambush force, killing 11 of the Japanese, before forming a harbour around the tanks for the night.

On the morning of the following day, 1 April, the Australians extended across the Kero Creek and advanced northward to the Puriata river, where they established a perimeter 1,000 yards (915 m) to the south of Slater’s Knoll. At about 12.00, the 6th Field Artillery Regiment, whose gunners had been converted to the infantry role, set up a form-up position 55 yards (50 m) to the front of the Australian company holding the perimeter to the south of Slater’s Knoll at a location the Japanese called Pain, and which lay along the Buin Road. Before the attack could be launched, the Australians detected the Japanese movement and began firing on them. Shortly after 12.40, the Japanese gunners launched their assault. Firing their weapons from the hip, they rushed the Australian position, pushing the defenders back, leaving dead and wounded behind. That evening, the Australians launched a series of determined counterattacks in order to re-establish contact between Slater’s Knoll and the forward companies, but these were driven back.

Meanwhile, the 23rd Regiment, which had advanced on the right flank from Barara attempted to move on Slater’s Knoll from the north. Finding its way blocked by wire obstacles and lacking the equipment necessary to remove them, the regiment was forced to halt its advance and cover the flank of Colonel Toyoji Muta’s 13th Regiment, which was attempting to launch an assault across the Puriata river. At this point, the river is about 110 yards (100 m) wide and 3 ft 4 in (1 m) deep, while the opposite bank is about 3 ft 4 in (1 m) high. The crossing began at 03.00 on 1 April, and the three battalions found the going difficult. When they were halfway across, the two forward battalions were spotted by the Australians in the moonlight and engaged with machine gun and rifle fire. The Japanese deployed quickly and attempted to launch an attack, but in the confusion the 1/23rd Regiment was cut off from the rest of the assault force. With the arrival of daylight, the Australians were able to call down mortar fire on the Japanese, causing considerable casualties. Running short of ammunition and having suffered considerable losses, the 23rd Regiment was forced to withdraw. The 1/23rd Regiment remained cut off until the evening, when it was able to re-establish contact with the other two battalions and report to the divisional commander that the attack had been unsuccessful.

On 2 April, the Japanese divisional commander, Akinaga, recalled the 13th Regiment and 23rd Regiment back across the Puriata river to a position lying to the south-west of the Pain feature. At this point, the Japanese made the decision to launch another attack to take Slater’s Knoll and eliminate the Australian forces in the area surrounding the Puriata river ford. The attack was scheduled for 5 April, and over the course of the following three days, contact was minimal except for a few minor skirmishes, and small groups of Japanese were seen by the Australians around Barara, Slater’s Knoll and Mosigetta. As further intelligence reports came in, it became clear to the Australian commanders that the Japanese were about to deliver the main attack.

On the night of 4/5 April, the Japanese launched a heavy bombardment of the Australian artillery battery, while communications between the headquarters of the 7th Brigade and that of the 25th Battalion were severed, as too were the lines from the 25th Battalion’s headquarters to its forward companies. Suspecting that the attack was now about to begin. runners were sent and the men defending Slater’s Knoll, now a force of just 129 men of B Company, stood-to.

At 05.00 on 5 April, the Japanese launched an attack from the north, and followed almost simultaneously with a stronger attack from the south-west. For the next 80 minutes, between 900 and 1,100 Japanese troops of the 13th Regiment and 23rd Regiment attacked in waves. The attack overwhelmed the Australian forward positions but, finding its way blocked by wire obstacles and lacking any equipment to deal with them, the Japanese attack stalled. The situation was quickly turned back in favour of the Australians, and the Japanese were cut down by the fire of the artillery and well-sited machine guns. Another attack was launched by a smaller Japanese force along the Buin road against two of the forward companies positioned there. This attack too was beaten back. After regrouping, the surviving members of the 13thy Regiment began preparing for a final charge, determined to fight to the last man. Muta, the regimental commander, wrapped his unit’s banner around his waist and armed himself with a grenade with which to blow himself up during the assault. Elsewhere, the remaining officers of the 23rd Regiment made a pact to hurl themselves against the Australians in a final suicidal charge. Nevertheless, Akinaga, realising that further assaults were futile, finally despatched an order to cancel the attack. At about 08.30, the Australians realised that the Japanese had decided that they had had enough. Just after 12.00. two tanks were send forward from the 'B' Echelon’s perimeter, escorted by a company of the 61st Battalion.Reaching the knoll by 13.45, the tanks were used as fire support as the Australians began mopping-up operations, flushing out small groups of survivors all round the position.

On the day following the final Japanese assault, 292 dead Japanese were counted around B Company’s perimeter. In total, over the course of the battle the Australians counted 620 Japanese dead and estimated that another 1,000 had probably been wounded. Information obtained from captured Japanese clarified that the Australians had come up against a force of at least 2,400 men. The commander of the 23rd Regiment, Lieutenant Colonel Kawano Koji, was among the dead, as was a senior divisional staff officer, Lieutenant Colonel Honda Matsuo. Four wounded Japanese were also taken prisoner. The Australian casualties included 10 officers and 179 other ranks killed or wounded, and another 81 Australians had been evacuated as a result of non-combat issues. Wartime Japanese casualty estimates vary from these figures, however, claiming that the Japanese had killed 1,800 Australians, while placing their own losses at 280 killed and 320 wounded.

The result of the 'Battle of Slater’s Knoll' exercised a profound effect on the Japanese commanders. According to one historia, Kanda had been so certain of victory that he had developed no plans for subsequent activities. After the battle, the Japanese forces in the area pulled back to the Hongorai river where, as a result of their heavy losses, they were reorganised with several infantry, engineer and artillery units amalgamated. Lacking food, suffering the effects of disease, and isolated from home, the Japanese suffered a heavy decline on morale, many of the men realising that the war was lost. There was an increase in desertions at this time, although the vast majority continued to fight.

Tactically, the battle proved to the Australian high command how effective armour could be when used in close co-operation with infantry and in protecting the lines of communication. The use of machine gun carriers to carry out the wounded had also been quite effective. While the Japanese had fought with their usual bravery, continuing the assault even when it was clear it had failed, ultimately the battle proved to be a decisive Australian victory; however, James notes that poor planning and unreliable communications had also contributed to the result. Long goes on to argue that if the Japanese commander had been more flexible in his tactics, the result might have been different. The Australian right flank had been vulnerable, but Akinaga did not exploit this opportunity.

There were tactical deficiencies as well, particularly the predictability of cutting Australian signals lines just before an attack and bunching together in tight groups during an assault, allowing the Australians to devastate attacks with their machine guns. McKinna had deployed his companies widely and effectively, and himself went forward to direct the fighting at several points. Fields of fire had been cleared, perimeters set and patrolling had been maintained. Nevertheless, the Australians had also failed to seize opportunities as they emerged, for after the battle they did not immediately follow up their success. After the failure of the final assault, the Japanese had possessed in the immediate vicinity no forces capable of resisting any follow-up forces that the Australians might have been able to bring forward, and it is possible that an advance could have been carried all the way to the Hongorai river. As it was, therefore, the Australian line of supply was stretched and a pause was required before the advance could be pressed in earnest.

In the days immediately after the battle, the Australians continued to carry out patrolling operations around the knoll. On 7 April, a patrol of the 9th Battalion clashed with a Japanese force of about 30 men, and in the ensuing engagement four Japanese were killed, while a patrol of the 61st Battalion on 8 April killed another five. On 13 April, the order was given that the 7th Brigade would be gradually withdrawn and relieved by the 15th Brigade. Shortly after this, the 58th/59th Battalion replaced the 25th Battalion on Slater’s Knoll, while the 24th Battalion resumed the advance along the Buin road on 17 April and subsequently took part in the fighting along the Hongorai river. The 9th Battalion was not withdrawn immediately, however, and remained in the northern area until it handed over to the 57th/60th Battalion early in May. The fighting on Bougainville continued throughout April as the Japanese resisted the Australian advance and went on until July when the fighting petered out as a result of heavy rain and consequent flooding.