The 'Battle of Piva Forks', which is also known as the 'Battle of the Numa-Numa Trail', was an engagement between Japanese and US forces during the Bougainville island campaign that followed the US 'Cherryblossom' landings (18/25 November 1943).

Taking place on Bougainville island, part of the Solomon islands chain in the South Pacific, the battle pitted troops of the US Marine Corps and US Army against forces of the Imperial Japanese army, and occurred within the context of the US expansion of its beach-head round Torokina on the western side of the island.

In response to the US expansion of its lodgement, the Japanese placed roadblocks along the main axes of advance in an effort to delay the US forces. After finding their advance toward the Piva river checked near the junction of the Numa-Numa and East-West Trails, the US forces sought to remove the obstacles by force. After they had repelled the initial US attack, the Japanese counterattacked before the US Marines overcame this reverse and continued their advance toward two forks in the Piva river. By 26 November the battle had subsided following the US capture of a knoll overlooking the East-West Trail. This represented the last of the significant features to the west of cape Torokina and the conclusion of the battle marked a temporary end to significant Japanese opposition to the US lodgement round Cape Torokina.

On 1 November 1943, Major General Alan H. Turnage’s US 3^rd Marine Division had landed at Cape Torokina near Empress Augusta Bay on the western coast of Bougainville. The landings were undertaken as part of Allied 'Cartwheel' efforts to isolate and reduce the main Japanese base around Rabaul on New Britain island. On Bougainville, the US troops were opposed by Lieutenant General Harukichi Hyakutake’s Japanese 17^th Army, which was part of General Hitoshi Imamura’s 8^th Area Army based at Rabaul. The main infantry forces opposing the marines on Bougainville were of Lieutenant General Masatane Kanda’s 6^th Division.

After the US landing, a counter-landing by Japanese troops was turned back in the area of the Koromokina Lagoon, while an overland assault was turned back along the Piva Trail. After this, the beach-head was slowly expanded by the marines while work began to construct several airfields, from which US air power would be projected toward Rabaul. After the 'Battle of the Coconut Grove' on 13/14 November, US patrols reported sporadic contacts with Japanese forces, and documents recovered from the body of a Japanese officer killed in an ambush provided the US forces with details of the Japanese dispositions in the area, showing that a roadblock had been established by elements of the 23^rd Regiment on both the Numa-Numa and the East-West Trails. Major General Roy S. Geiger, commander of the I Marine Amphibious Corps, planned the expansion of the beach-head perimeter established round Torokina to a new defensive line farther inland designated as 'Easy'. Geiger specified that the line was to be obtained by 20 November 1943. On 18 November, US patrols discovered a Japanese roadblock on the Numa-Numa Trail about 1,000 yards (915 m) in front of the beach-head perimeter, while another patrol found a roadblock mid-way between the two branches of the Piva river along the East-West Trail.

With the roadblocks preventing the advance to the 'Easy' inland defence line, the US forces initiated preparations for the removal of these roadblocks. The 3^rd Marine Raider Battalion was attached to Colonel George W. McHenry’s 3^rd Marines for the attack on the East-West Trail roadblock, while the 3/3^rd Marines, under the command of Lieutenant Colonel Ralph M. King, was detached to attack the Numa-Numa Trail roadblock.

On 19 November, the 3/3^rd Marines, accompanied by light tanks,took up positions in front of the 129^th Infantry. A barrage by the artillery of the 12^th Marines preceded the attack, and after the barrage’s end the 3^rd Battalion outflanked the Japanese position, routing the defenders and forcing them out of their positions. The bodies of 16 Japanese dead were found and almost 100 foxholes were located: the extent of the position indicated that it had been occupied by a force of at least reinforced company strength. After this, a roadblock and a defensive perimeter were established at the junction of the Numa-Numa Trail and the Piva river to defend against any Japanese counterattack. Meanwhile, the 1/3^rd Marines and the 1/21^st Marines advanced to the 3/3^rd Marines' roadblock. The 3^rd Marine Raiders also moved forward for support and the 2/3^rd Marines, under the command of Lieutenant Colonel Hector de Zayas, moved behind the Numa-Numa Trail roadblock and was sporadically shelled by Japanese 90-mm (3.54-in) mortars.

During the morning of 20 November, the Japanese counterattacked in an attempt to outflank the marine positions along the Numa-Numa Trail. The counterattack was driven back, the Japanese then taking up positions and harassing the marines with sniper and mortar fire. The 3/3^rd Marines advanced toward the two forks of the Piva river in order to clear the threat, and two light tanks were disabled during the close fighting along the trail. The 3^rd Marine Raiders assumed positions to cover the gap between the 129^th Infantry and the 3^rd Marines as the length of the front expanded.

The 2/3^rd Marines advanced across the west fork of the Piva river to capture the Japanese positions between the river’s two forks. A river crossing was made using a bridge of mahogany, hastily constructed by engineers. As they moved forward under light opposition from scattered snipers and several machine gun nests, the US force discovered that the Japanese outposts had been abandoned after they had been booby-trapped. The 2/3^rd Marines set up positions astride the East-West Trail between the two river’s forks, and the 21^st Marines took up blocking positions behind the 3^rd Marines.

Late in the afternoon of 20 November, it was discovered that a small, 400=ft (120-m) high ridge (later named Cibik Ridge) would provide observation of the entire Empress Augusta Bay area, and also dominated the East-West Trail and the Piva Forks area. A platoon under the command of 1^st Lieutenant Steve J. Cibik was ordered to occupy the ridge with detachments of signals and a section of heavy machine guns. The advance began with a struggle up the steep ridge late in the afternoon, with signal wire being reeled out as the platoon climbed the ridge. Reaching the summit just before sunset, the Americans set about the establishment of defensive positions, with machine guns sited along likely axes of attack.

With the break of day on 21 November, it was discovered that the crest of the ridge was a Japanese outpost position, used during the day as an observation post and abandoned at night. Japanese soldiers approaching to take up positions were engaged and fell back to flee down the hill. After regrouping and being reinforced, the Japanese launched several attacks on Cibik’s platoon, which itself had been reinforced by more machine guns and mortars, and was thus able to hold the crest in the face of fanatical Japanese attacks.

Geiger’s expansion of the beach-head to 'Easy' inland defence line began at 07.30 on 21 November with the gradual widening of the perimeter, allowing the 21^st Marines, under the command of Colonel Evans O. Ames, to set up positions between the 3^rd Marines and 9^th Marines. The 1 and 3/21^st Marines led the advance, crossed the Piva river without difficulty and by a time early in afternoon had reached the designated defence line. On the extreme left flank, a reinforced platoon was attacked by a strong Japanese patrol, but was able to repulse the attack with heavy losses to the Japanese. Documents discovered on the body of a Japanese officer provided the marines with details about the state of Japanese defences ahead of them.

Resistance to the 3^rd Marines' advance was strong, with all three marine battalions engaged with the Japanese. Having crossed the Piva river without trouble, the 3/3^rd Marines advanced toward a slight rise and, as the leading scouts came over the top of this ridge, the Japanese opened fire from reverse-slope positions. Pinned down, the scouts held their positions while the rest of the battalion moved forward with a strong charge over the ridge and cleared the area of Japanese. Then 90-mm (3.54-in) mortar fire rained down on the Americans, who sheltered in the foxholes that the Japanese had dug throughout the area. Suffering a number of casualties, the 3/3^rd Marines established itself in a defensive position for the night.

The 2/3^rd Marines made a reconnaissance in force ahead of the 1/3^rd Marines and came across a strong Japanese position, with about 18 to 20 pillboxes, astride the East-West Trail near the east fork of the Piva river. An attack against the roadblock managed to penetrate through the first line of bunkers after heavy close-quarter fighting, but could make no further headway. Attempting to flank the Japanese positions to relieve the intense fire directed at Company G, Company E was driven back by the Japanese defenders.as the Japanese position had been created in depth, the US battalion commander, de Zayas, ordered his men to pull back so that an artillery barrage could be brought down to reduce the Japanese defences. The withdrawal was completed despite determined efforts by the Japanese to prevent the disengagement. After the 2/3^rd Marines had re-entered the lines of 1/3^rd Marines, the Japanese attempted a pincer envelopment of the position held by the 1/3^rd Marines, now under the command of Major Charles J. Bailey. The Japanese attacked along the obvious routes of approach down the East-West Trail, and the attack was beaten off by the machine guns sited along this route. One machine gun crew killed 74 out of 75 of the Japanese attackers within 20 to 30 yards (18 to 27 m) of the gun. The 1/3^rd Marines then extended its line to the left toward the ridge occupied by Cibik’s platoon.

The 9^th Marines crossed the Piva river and took up defensive positions about 1,000 yards (915 m) to the east of the Piva river between the 21^st Marines and the beach. The 129^th Infantry also moved forward another 1,000 yards (915 m) with encountering any opposition. As a result of inaccurate maps, a gap developed between the 21^st Marines and 3^rd Marines. At this point, the US forces paused in their perimeter expansion as Japanese forces were well dug-in to their front. Plans were drafted for an attack on the Japanese defences, which faced to the south as a result of the trail’s twistiness, from west to east and preparations for an assault on 24 November were undertaken. As armour and equipment was moved into positions behind the 3^rd Marines, the roads were extended by engineers and 'Seabee' construction units, which pushed them forward as close as possible to the Piva river forks. Operating under fire from Japanese snipers and mortars, the engineers and 'Seabees' built bridges across the Piva river, and large quantities of ammunition, rations and medical supplies were sent forward via a relay system to the front line, while the wounded were tended at a medical station set up close to the end of the road.

The 2^nd Marine Raider Battalion was attached to the 3^rd Marine Regiment during 22 November and relieved the 3^rd Battalion, 3^rd Marine Regiment, which moved into reserve.[17] The 3^rd Marine Regiment’s positions were: 1^st Battalion, 3^rd Marine Regiment in front, with 3^rd Battalion, 3^rd Marine Regiment on the left of the trail and the 2^nd Battalion, 3^rd Marine Regiment on the right.[18] Cibik’s reinforced platoon holding the ridge in front of the perimeter, was reinforced with a company of Marine Raiders and a platoon from the 3^rd Marines Weapons Company.[19]

On 23 November, the 12^th Marines' artillery observers moved to the crest of the ridge occupied by Cibik’s platoon in preparation for the attack scheduled for the following day. As the US artillery and mortars were registered on the area in front of their position, the front-line marines used coloured smoke grenades to mark their positions. The Japanese, however, were using the same colours to mark US positions and, as a result, the Japanese artillery were able to range on the forward marine positions with several rounds. Japanese long-range guns also shelled Torokina airfield and a number of tank landing ships which were unloading near Cape Torokina. In response, US forward artillery observers co-ordinated fire missions against several positions where they believed that the Japanese guns might be located and shortly after this the Japanese artillery fire ended. Nevertheless, the Japanese artillery fire worried US commanders already concerned about reports that the size of the Japanese forces defending the area around the village of Kogubikopai-ai was between 1,200 and 1,500 men.

At 08.35 on 24 November, the seven battalions of the US artillery group opened fire on the Japanese positions in front of the 3^rd Marines. The bombardement lasted for more than 20 minutes, and in this time more than 5,600 75- and 105-mm (2.95- and 4.13-in) howitzer shells dropped on the Japanese positions. Smoke shells were also fired into the hills to the east of the Torokina river to reduce the ability of the Japanese to observe the marine positions.

The 2 and 3/3^rd Marines began to move forward to the front line in preparation of the launch of the attack. Tanks, which were assigned to a secondary role in the attack, drove toward the front line to move into support positions. Before 09.00, the 1/3^rd Marines saturated the area with close-in mortar concentrations and sustained machine gun fire to prevent the Japanese from seeking protection close to the marine lines. Just before the start of the attack, Japanese artillery began a counter-barrage which blasted the marine lines, pounding the positions of the 1/3^rd Marines and the assembly areas of the assault battalions: the extreme accuracy of the Japanese artillery fire threatened to force a halt on the attack plan. The value of the forward observers on the ridge was exploited to discover the location of the Japanese battery, and counter-battery fire was requested. Communications were lost but the break in the signal wire was found and repaired.

The Japanese battery was located on the forward slope of a small coconut grove several thousand yards from the Piva river. Counter-battery fire by the 155-mm (6.1-in) howitzer battery of the US Army’s 37^th Division began to explode around the grove, the fire was adjusted quickly by direct observation, and shortly afterwards the Japanese battery was knocked out of action. While the artillery duel was under way, the 2 and 3/3^rd Marines began forming into attack formation behind the line of departure. The preparatory fire came to an end at 09.00, and at that time the two attack battalions advanced through the lines of the 1/3^rd Marines.

Moving through the artillery preparation zone, the marines saw that the Japanese forward positions had been neutralised. The shattered and cratered jungle with destroyed Japanese defensive positions was crossed without opposition. branches. Soon after this, however, the lull ended and the surviving Japanese opened fire. Japanese artillery shells burst along the line, traversing the front of the advancing marines, and was complemented by extremely accurate 90-mm (3.54-in) mortar fire in striking the attacking companies and causing significant casualties. The 2/3^rd Marines suffered 70 losses in an advance of only 250 yards (230 m). During the morning, the marines were forced to undertake eight water crossings of the same stream as it zigzagged across their axis of advance. At each bend in the stream, the Japanese had established a number of pillboxes in triangular formations with interlocking fields of fire, and before the Americans could advance they had to neutralise each of these. Brought forward by engineers attached to the assault companies, flamethrowers were employed in this task but, aware of the capabilities of the flamethrowers, the Japanese concentrated their fire on the engineers, inflicting heavy casualties on them as they attempted to get close enough to direct the flame into the bunkers.

Advancing to the left of the East-West Trail, the 3/3^rd Marines encountered less resistance, and was able to continue its advance without pause. The dazed and shocked Japanese survivors of the bombardment were killed before they could recover from the effects of the artillery fire. The 3/3^rd Marines had moved nearly 500 yards (455 m) before the Japanese could organise a desperate counterattack, which was repelled. Without stopping, the 3/3^rd Marines went straight through the Japanese flanking attempt and fought a violent hand-to-hand and tree-to-tree struggle that completely destroyed the Japanese force.

By 12.00, the initial US objectives had been reached and the attack was checked to allow reorganisation and to re-establish contact between units. After a short time, the attack was resumed once again toward the final objective some 400 yards (365 m) farther forward. Another artillery barrage was called down in front of the advancing marines and 81-mm (3.19-in) mortars covered the advance. As this movement began again, the Japanese called in their own mortars to seek out the US mortar crews, but the infantry attack continued, while overhead supporting and defensive fire was exchanged.

The 3/3^rd Marines came under fire from Japanese machine guns and rifles from positions on high ground bordering a swampy area, and this Japanese fire raked through the battalion, forcing the marines to seek cover in the knee-deep mud and slime. On the extreme left, Company L was under heavy fire and was reinforced quickly with a platoon from the reserve unit, Company K. Company L managed to fight its way through heavy Japanese fire to the foot of a small knoll.Company I, with the battalion command group attached, was diverted to help and together they were able to rush and capture the rising ground. After the Japanese had been cleared from the position, the battalion established a defensive perimeter and waited for 2/3^rd Marines to move into position beside it.

The advance of the 2/3^rd Marines was slowed by the arrival of strong Japanese reinforcements as it closed on the objective. Calling down 60- and 81-mm (2.36- and 3.19-in) mortar fire in front of their positions, the marines regained the initiative and moved forward. Although the remaining Japanese made a determined final stand on the objective, the marines were able to push ahead and as the resistance subsided the battalion mopped up before establishing a defensive position for the night. In the US rear area, behind the 2 and 3/3^rd Marines, sporadic fighting continued during the night as small pockets of Japanese troops bypassed during the US assault, attacked the marines' supply lines, ambushing stretcher bearers and troops bringing up ammunition.

The advance to the 'Easy' inland defence line had been carried out successfully by the US forces. Casualties during the battle reflected the intensity of the fighting: at least 1,071 Japanese had been killed while the marines' casualties had amounted to 115 dead and wounded. Such was the intensity of the fighting that the 23^rd Regiment was almost completely destroyed. Such was the intensity of the fighting that during the attack on 24 November, the supporting US artillery fired a total 62 different fire missions including the attack’s opening bombardment: of these, 52 were in general support of the marines, while another nine were fired to support the 37^th Division, which was also expanding its perimeter on the marines' left. Throughout these missions, more than 7,300 rounds were fired: 4,131 rounds of 75-mm (2.95-in), 2,534 rounds of 105-mm (4.13-in) and 688 rounds of 155-mm (5.91-in) ammunition.

As 24 November was Thanksgiving Day, the beach-head had taken delivery of a large number of turkeys, and the divisional cooks roasted the entire shipment and packed the turkeys for onward distribution to front-line units. The 3^rd Marines, exhausted from the fighting and below establishment as a result of casualties and disease, were relieved the following day by the three battalions of the 9^th Marines. Meanwhile, the US advance continued throughout the day until it met heavy Japanese resistance at a knoll dubbed 'Grenade Hill', which was eventually captured by middle of the morning of 26 November. Over the course of 25/26 November, fighting around the knoll resulted in a further 32 Japanese killed, while US casualties amounted to five killed and 42 wounded.

By the end of the Piva Forks fighting, US forces were in control of the majority of the key terrain features to the west of the Torokina river. This reduced the Japanese forces' ability to harass the US beach-head around Torokina and as a result, despite the fact that the fighting on Bougainville island continued with notable actions being fought at Koiari, Hellzapoppin Ridge and Hill 600A, the Japanese were essentially a spent force on Bougainville island. Indeed, it was not until March 1944 that the Japanese were able to make any real attempt at counterattacking when they launched a major offensive on the US perimeter with 15,000 troops of the 6^th Division and 17^th Division, who attacked to the north and west from Buin and Numa-Numa.