The Preliminary Operations
The Invasion of Bougainville
The First Japanese Counterattack
Expanding the Beachhead
The March 1944 Japanese Offensive
Operation Cherryblossom, the invasion of Bougainville (1 November 1943-March 1944) was the last major operation during the Solomon Islands campaign and saw the Americans occupy a secure bridgehead on an Island that the Japanese had decided to make a bastion of their defensive line.
After the end of the fighting on Guadalcanal the American's next target was the New Georgia group. Operation Toenails, the invasion of New Georgia, began with the capture of Rendova Island on 30 June. The battle for Munda, on New Georgia Island, lasted until 5 August. The Japanese were forced off Arundel Island by 20 September, and Vella Lavella by 6 October. The isolated base on Kolombangara was evacuated by 2-3 October, ending the New Georgia campaign.
The main American target was the important Japanese base at Rabaul. Originally they intended to invade Rabaul, but it was soon realised that this would be very costly and they decided to bypass Rabaul, cut it off from the rest of the Japanese Empire and leave it to wither. Both plans required the capture of bases on or near to Bougainville, from where American ground-based fighters could reach Rabaul.
The Americans produced a series of plans for the Bougainville campaign. At first they intended to land on the southern part of the island and capture the Japanese bases at Buin and Faisi. In late July, with American troops bogged down near Munda, Admiral Halsey began to doubt the wisdom of another attack on entrenched Japanese troops and came up with a second plan. This time the Americans would seize a number of islands around Bougainville, most importantly Ballale and the Shortland Islands. Artillery would be placed on the Shortlands and airfields on other islands, and the Japanese troops on Bougainville Island would be left to wither on the vine. This plan was in place throughout August, but a third plan was developed in September.
This time the Americans would capture the Treasury Islands and Choiseul Bay and use them as air bases from where southern Bougainville and the Shortland Islands could be neutralised. This would be followed by a landing on either the east or west coast of Bougainville if required. Halsey approved this scheme, but MacArthur worried that it would delay the construction of fighter bases within range of Rabaul for too long. He suggested a landing at Empress Augusta Bay, on the west coast of Bougainville.
Admiral Halsey accepted MacArthur's concerns, and on 22 September he issued a new set of orders. Two different plans were to be created. The first called for an invasion of the Treasury Islands, followed by a landing at Empress Augusta Bay. The second was for the occupation of the Treasury Islands and Choiseul Bay, followed by a landing on the east coast of Bougainville. A series of reconnaissance parties were sent out to examine the two possible landing places.
The east coast was ruled out, but Empress Augusta Bay looked promising. Soil samples suggested that it was a suitable area for an airbase, and its isolated position meant that the Japanese would be unable to mount a rapid counterattack. The area was believed to be garrisoned by around 1,000 men. Disadvantages were the lack of sheltered landing beaches, and the short distance to the Japanese airfields elsewhere on the island and on Rabaul. On 1 October 1943 Halsey informed MacArthur that he had decided to land at Empress Augusta Bay in one month's time, on 1 November 1943.
Admiral Halsey had fewer resources for the invasion of Bougainville than had been available on New Georgia. He had eight transport ships and four cargo ships, enough to carry one reinforced division. His main naval force was Task Force 38 (Rear Admiral Frederick C. Sherman), with carriers Saratoga and Princeton, two cruisers and ten destroyers. Nimitz promised to send a second carrier task force after 8 November and more reinforcements if needed.
The ground forces came under the overall command of General Vandegrift, commander of the I Marine Amphibious Corps. He had the 3rd Marine Division, 37th Infantry Division, 8th Brigade Group 3rd New Zealand Division, 3rd Marine Defence Battalion, 198th Coast Artillery Regiment (Antiaircraft), 2nd Provisional Marine Raider Battalion and the 1st Marine Parachute Battalion as well as a number of engineer and support units. Command of the troops within the Bougainville beachhead would be held by General Turnage, commander of the 3rd Marine Division.
D-Day for the main invasion was set as 1 November 1943. Two preliminary operations were to begin on 27 October. Operation Goodtime, the invasion of the Treasury Islands, was intended to provide the Americans with a site for new airfields nearer to Bougainville. Operation Blissful was to be a diversionary raid on Choiseul Island, which was intended to distract the Japanese.
The island of Bougainville is 125 miles long along its northwest-to-southeast axis. In 1943 its mountains were jungle covered, and crossed by native trails but no modern roads. The heart of the island consists of the Emperor and Crown Prince mountain ranges, in the north and south of the island respectively. The west coast was bordered by a swampy coastal plain, cut by a number of small rivers. Empress Augusta Bay is in the centre of the west coast, and was very isolated from other parts of the island.
To the north of the main island is Buka Island; while to the south-east are the Shortland Islands, with the Treasury Islands a little further to the south.
There was a good harbour at Buka, along with a Japanese airfield. There was another airfield at Bonis, on the mainland side of the Buka Passage. On the east coast there was a harbour at Numa Numa and Tenekau and airfields at Tenekau and Kieta. The main Japanese base was on the flatter ground at the south-eastern end of the main island, where they had airfields at Kara and Kahili, close to the harbour at Tonolei. There was another airfield at Ballale in the Shortland Islands, and there were also good harbours and seaplane bases in the Shortlands. The west coast lacked harbours or airfields.
The defenders of Bougainville were commanded by General Hyakutake, commander of the 17th Army. He had been responsible for the unsuccessful campaign on Guadalcanal. The Americans believed that he had 37,500 soldiers and 20,000 sailors. Most of the soldiers (some 25,000) were in the south, either around Buin or on the Shortland Islands. Another 5,000 were on the east coast and 5,000 more at Buka and Bonis in the north. Emperor Augusta Bay was believed to be defended by no more than 1,000 men.
The American estimate was fairly accurate. Hyakutake had 26,800 men in the Buin and Shortlands area, 4,000 at Kieta on the east coast and 6,000 at Buka and Bonis. His main unit was the 6th Division, which had taken part in the sack of Nanking in 1937, and three battalions from the 4th South Seas Garrison Unit. Four battalions from the 17th Division arrived in November, and made up part of the 6,000 strong force in the north.
A key role in the preparation for the invasion of Bougainville was played by the Allied air forces. The first in a long series of heavy raids on Bougainville itself began on 6 July 1943, and continued throughout the fighting in New Georgia.
The end of the campaign in New Georgia meant that Rabaul was now within range of several Allied airfields. The first big allied raid came on 12 October, when 349 aircraft took part in the attack (87 heavy bombers, 114 B-25 Mitchells, 125 P-38 Lightnings and 12 Beaufighters. A second big raid was planned for 18 October but bad weather intervened and only 54 B-25s reached their targets. There were further raids on 23, 24, 25 and 29 October and these made it much harder for the Japanese on Rabaul to intervene during the Bougainville landings on 1 November. Another large attack was launched on 2 November, and this time the Allied airmen tangled with Japanese naval fliers who had just arrived from Truk, again preventing them from attacking the beachhead at Bougainville.
Nearer to Bougainville the South Pacific aircraft of General Twining focused on Japanese bases at Kahili, the Shortland Islands, Ballale, Kieta and Buka. A continuous series of attacks began on 18 October, and all of the Bougainville fields had been knocked out by 1 November.
On 1 November itself the Americans launched two naval attacks on the airbases at Buka and Bonis. At 0.21am on 1 November a naval task force arrived offshore and fired 300 6-inch and 2,400 5-inch shells at the Japanese bases. They then moved off to attack the Shortland Islands. The second attack came from carrier-borne aircraft (the first time the US carriers had come within range of Rabaul and their first use in support of the fighting in the Solomons since Guadalcanal). The carrier force carried out a series of air attacks on Buka and Bonis on 1-2 November.
The Preliminary Operations
Admiral Wilkinson, commander of Task Force 31, split his fleet into two. He retained command of the northern force, while the southern force (Admiral Fort) was allocated to the Treasury Islands and Choiseul raids.
The invasion of the Treasury Islands (Operation Goodtime) was carried out by the 8th Brigade of the 3rd New Zealand Division with support from American artillery and engineers. The only Japanese resistance came on Mono Island, the northern and largest of the Treasury Islands. Even here the Japanese were driven away from the beach by noon on the day of the invasion (27 October), and by 12 November all resistance was over.
Operation Blissful - The Choiseil Raid (27 October-4 November 1943) - was a qualified success. A small force of Marine paratroops was landed by sea on the south-western coast of the island. They carried out a number of raids on nearby Japanese positions, but by 3 November it was clear that the Japanese had realised how small the American force actually was, and that they were preparing a large-scale attack. On 4 November the American troops were safely evacuated. The raid may have helped convince the Japanese high command that the next American target would be the southern end of Bougainville, but the main invasion was so close to the diversionary raid that the Japanese had little time to react to it.
The Invasion of Bougainville
The main invasion force was carried by the northern force of Task Force 31, which remained under the direct command of Admiral Wilkinson. This force included all of the available transport and cargo ships, which were to carry most of the 3rd Marine Division and its supporting units. The 37th Division, which was to make up the second wave of the invasion, would be carried to Bougainville in the same ships.
Admiral Wilkinson set H-Hour for the invasion as 7.30am on 1 November 1943, so that his transport ships could navigate the uncharted waters of Empress Augusta Bay in daylight. A pre-invasion naval bombardment began at 5.47am, and at 7.26 the landings began. A total of 7,000 men were to land in the first wave, using eleven beaches in Empress Augusta Bay and one on nearby Puruata Island. The landings were supported by a naval bombardment and by American torpedo bombers.
The Japanese had a small garrison at Cape Torokina and on Puruata Island. The island was defended by a single platoon, while Cape Torokina was held by 270 men from the 23rd Infantry Regiment (some from the 1st Battalion and some from the Regimental Gun Company). They had built at least 25 pillboxes and had a single 75mm gun.
Most of the landing beaches were undefended, but the troops landing closest to Cape Torokina, at the southern end of the landing area, faced a hard fight. The area wasn't cleared of defenders until 11.00am, and the Americans lost 78 dead and 104 wounded during the fighting. Puruata Island was cleared by 2 November, but the main beachhead continued to come under sniper attack.
The main threat to the American beachhead came from the air. The first Japanese attack came at around 8am. A New Zealand fighter squadron intercepted the incoming Japanese fighters, while Marine fighters dealt with a second wave that included some bombers. The Japanese failed to sink any ships in this first attack, but did delay the landing by two hours.
A second air attack, of around 70 aircraft, arrived at around 1pm. Once again the Japanese were intercepted by Allied aircraft, but their arrival forced the transports to withdraw for a second time.
By the end of the day the Americans had landed 14,000 men and 6,000 tons of supplies on the narrow spit of land between the high tide mark and a large swamp that had been discovered just behind the beach. A fairly chaotic situation soon developed, with supplies piled up the narrow beach, and other supplies diverted to Puruata Island. On the night of 1-2 November a number of Japanese troops attempted to infiltrate into the American beachhead and a casualty clearing station was attacked by a slightly larger force.
Early on 2 November the Japanese navy attempted to intervene. By chance a force of heavy cruisers had been visiting Rabaul from Truk at the time of the invasion, and they were combined with locally available ships to form a dangerous task force. Admiral Merrill's Task Force 39 had to quickly return to intercept them. The resulting Battle of Empress Augusta Bay (2 November 1943) was a night-time victory for the US Navy. The Japanese lost a cruiser, and at 3.37am, believing that he had sunk three American cruisers, Admiral Omori, the Japanese commander, ordered his fleet to withdraw. The first Japanese naval threat to the American beachhead had been repulsed.
On 2 November the four cargo ships were unloaded. Resistance ended on Puruata Island and the main beachhead was extended. On 3 November Torokina Island was occupied, but the few Japanese troops known to be on the island escaped before the landing.
The main immediate threat to the beachhead continued to come from the sea. On 4 November Admiral Kurita arrived at Rabaul with seven heavy cruisers, one light cruiser and four destroyers. Admiral Halsey had a difficult decision to make - his main surface force had been heavily engaged on 1-2 November and would have been outnumbered anyway. This left Admiral Sherman's Task Force 38, built around the carriers Saratoga and Princeton. So far Halsey had avoiding using his carriers against Rabaul, but this was an emergency.
Halsey expected to suffer heavy losses in this attack, which took place on 5 November, but he was to be pleasantly surprised. The carriers were supported by ground-based aircraft from New Georgia, so they were able to make a full strength attack on Rabaul. Ninety seven aircraft were involved in the attack, which inflicted heavy damage on three heavy cruisers, two light cruisers and two destroyers. Most of the Japanese fleet withdrew to Truk, apart from the heavy cruiser Maya and the light cruiser Agano, both of which were too heavily damaged to be moved. Only ten American aircraft were lost, but in return the threat to the landing force on Bougainville was eliminated and the Japanese never sent heavy ships back to Rabaul. The American carriers were attacked on 7 November, on their way out, but were undamaged.
By 5 November the Marines had secured a beachhead that was around 10,000 yards long along the beaches, and reached 5,000 yards inland. Most Japanese resistance had been eliminated, and preliminary work had begun on the construction of an airstrip at Cape Torokina. Captured documents suggested that the main Japanese threat would come from the south, and so the American right flank was strengthened.
The First Japanese Counterattack
Although many Japanese commanders still believed that the landings at Empress Augusta Bay were a diversion, with the main invasion still to come, General Hyakutake was still ordered to destroy the American beachhead.
The Japanese counterattack would involve two units. Two battalions from the 23rd Infantry, which were already on Bougainville, were sent cross-country from Buin in the south towards Cape Torokina. The 17th Division, based at Rabaul, was to provide a raiding force that would land on the coast just to the north of the American beachhead.
The first attempt to land this force was made on 1 November, but was abandoned after strong American naval forces were reported to be in the area. A second attempt was made on 6-7 November and this time around 475 men were successfully landed two miles to the north of the American beaches. There was an American platoon located on the Japanese landing beach, but at first they weren't sure who was landing.
The Japanese suffered from many of the same problems as the Americans. The heavy surf had disrupted their landing, and the thick jungle and swamps disrupted their efforts to organise. They quickly came under fire from the 3rd Battalion, 9th Marines (Lt Col Walter Asmuth Jr), who held the left of the American beachhead.
Soon after 8am around 100 Japanese troops took part in the first attack on the American positions. They were hit by heavy artillery fire and soon forced to dig in (using abandoned American positions). The Americans decided to launch an attack of their own, but the 3/9th soon ran into effective machine gun fire and their attack stalled. During the afternoon the 1st Battalion, 3rd Marines took over the attack. Their attack also failed, and a heavy artillery bombardment of the Japanese positions began.
A third American attack, this time by the 1st Battalion, 21st Marines, began on the morning of 8 November. This time there was little resistance, the prolonged artillery bombardment having done its job. 250 Japanese bodies were found. The survivors of the Japanese force retreated north, but were hit by dive bombers based at Munda and wiped out.
One of the problems for the Japanese attackers was that the second prong of the counterattack didn't have a much impact as the Japanese had hoped. As the Americans advanced toward a key trail junction (between the Piva and Numa Numa trails) they ran into intermittent Japanese opposition. The Marines established a roadblock on the key Numa Numa trail, and repulsed two attacks on the night of 5-6 November. A larger attack (of about battalion size) was defeated on 7 November.
The first large scale attack came on 8 November, and involved troops from the 1st and 3rd Battalions, 23 Infantry, which had finally reached the area. This battle continued on 9 November, when both sides launched attacks. The Japanese were finally forced to retreat soon after noon, and the Americans were able to resume their advance. The two days of fighting had cost the Marines 12 dead and 30 wounded and the Japanese at least 100 dead.
The first significant American reinforcements arrived on 8 November, when the 148th Regimental Combat Team, 37th Division, reached Bougainville. The 129th arrived on 13 November and the 145th on 19 November, completing the movement of the division. As the fresh troops arrived the 37th Division was allocated to the left (northern) side of the beach head and the 3rd Marines to the right (southern) side. The arrival of the first Army troops allowed the Marines to move more troops to their right flight to help with the fight against the 23rd Infantry. The reinforcements brought with them a change in the command structure. On 9 November General Vandegrift was replaced by Major General Roy S. Geiger as commander of I Marine Amphibious Corps. Geiger took direct command of the beachhead. General Turnage returned to command of the 3rd Marine Division while from 13 November General Robert Beightler took command of the Army sector.
By 11 November both flanks of the Japanese counterattack had been smashed. The Japanese had badly underestimated the scale of the American invasion, and their two uncoordinated attacks had never really stood much chance of success.
11 November also saw a heavy carrier attack on Rabaul, using the two carriers from the attack on 5 November and the newly arrived Essex, Bunker Hill and Independence. Once again the attacks were success. The new carriers were attacked as they withdrew, but once again the Japanese were unable to inflict any significant damage.
12 November saw two significant developments. The fighting on Mono Island on the Treasury Islands ended, securing the site of a planned American base. On the Japanese side Admiral Koga had used 173 carrier aircraft and 192 aviators to reinforce the forces at Rabaul, but by 11 November 121 of the aircraft had been los and 86 of the men killed. Admiral Koga realised that he needed the survivors for his carriers, and on 12 November they flew back to Truk. These aircraft losses played a part in the inactivity of the Japanese Combined Fleet when Admiral Nimitz invaded the Gilbert Islands later in November.
Expanding the Beachhead
One of the main purposes of the landings on Bougainville was to find a suitable site for airfields that could be used against Rabaul. US scouts found a suitable site at a coconut grove close to the Piva River, near the junction of the Numa Numa and East-West trails. The coconut grove was too far from the existing beachhead to easily be included within it, and so the Marine commanders decided to establish a separate outpost at the future airfield site. This task was allocated to the 21st Marines, but when they reached the trail junction on 13 November they found that the Japanese had already occupied it. It took two days to force the 23rd Infantry to retreat but by 15 November the area was secure and the main beachhead had expanded to include the coconut grove area.
The next serious fighting began on 17 November, as the Marines attempted to expand their control of the area around the River Piva. On 17 November the fighting was limited to clashes between patrols, but on 18 November the Americans found Japanese trail blocks around 1,000 yards beyond their lines. One of these roadblocks was cleared on 19 November, but on 20 November its former defenders launched their own counterattack. This was repulsed, and by the end of the day the Marines had crossed one branch of the Piva River and a platoon under First Lieutenant Steve J. Cibik had occupied a key ridge that overlooked the entire area.
This ridge turned out to be the site of a Japanese daytime outpost and on the morning of 21 November the returning troops were ambushed by Cibik's men. This first repulse was followed by three heavier attacks during 21 November, but the ridge was held. The same day also saw the start of a wider Marine offensive designed to reach a new perimeter line.
22 November was fairly quiet. The Americans realised that they had reached the main Japanese line, and thus needed to plan for a proper attack. 23 November was taken up by planning and an artillery duel, and the attack began on 24 November. The American attack was nearly foiled by accurate Japanese counter-artillery fire, before observers on Cibik Ridge spotted the main Japanese gun position and directed American fire onto it. The advancing marines captured all of their objectives on 24 November, and repelled the inevitable Japanese counterattack. On 25 November the Japanese managed to hold onto a key feature that the Marines named Grenade Hill, but on the following day the position had been abandoned. The Americans had reached their overall objective for the offensive, and fighting died down.
24 November was a notable day - the airfield at Cape Torokina wasn't yet ready for regular use, but on that day an aircraft made the first emergency landing on the field. That night also saw the last in the long series of naval battles in the Solomon campaign - the battle of Cape St George (24-25 November). This saw the US Navy inflict a crushing defeat on a force of destroyers attempting to bring reinforcements to Bougainville.
On 29 November the marines attempted to raid the Japanese supply base at Koiari, ten miles south along the coast from their beachhead. The raid was something of a fiasco - the marine paratroopers were landed from the sea right into the middle of a Japanese supply dump, and after coming under heavy attack all day had to be rescued with the help of three nearby destroyers.
9 December was a key day on Bougainville. On that day the fighter strip at Cape Torokina was declared to be ready for operations, and on the following day 17 F4U Corsairs of Marine Fighting Squadron 216 flew into the new airfield. The whole purpose of the Bougainville campaign had been to create airbases within fighter range of Rabaul, and this had now been achieved. On 17 December fighters from New Georgia used the Cape Torokina airfield during a raid on Rabaul, the first time the new base was used for its intended purpose.
The final major expansion of the beachhead during 1943 began in early December when the Americans decided to occupy a range of hills south of their existing lines. The Japanese had been using some of these hills as artillery positions, and they were yet another viewpoint over the beachhead. The hardest fighting in this area was for a 300foot long ridge with steep sides and a narrow crest. This ridge earned the name 'Hellzapoppin Ridge' after it became the site of fierce fighting. The Americans first discovered that the Japanese had defensive positions on this ridge on 7 December when they captured an operations map. A series of attacks were launched against the ridge, but its shape and position made it a difficult target for artillery. An air attack on 13 December was ineffective, but further attacks on 14 December and 15 December did better. A fourth attack on 18 December was followed by a final successful attack and the ridge fell. A few days later the nearby Hill 600-A also fell, and the fighting died down
This was just about the last fighting on Bougainville to involve the Marines. The 3rd Marine Division was wanted for the planned assault on Kavieng, and so it was to be replaced by the Americal Division. I Marine Amphibious Corps would be replaced by XIV Corps, and General Geiger by General Griswold. This last change took place on 15 December while the first troops from the Americal (the 164th Regimental Combat Team) arrived on 25 December. Part of the 3rd Marine Division left on the same ships. General Hodges arrived on 28 December and took over command of the eastern sector. The 182nd Regimental Combat Team replaced the 21st Marines and the 132nd Regimental Combat Team went into the line on 9 February. Finally the Americal Division's field artillery took over on 14 February.
On 30 December a second airfield was completed within the American perimeter. This was the Piva Uncle strip, close to the Piva River. Piva Yoke was completed on 9 January 1944 and the Americans were finally able to operate bombers from Bougainville. The full scale neutralisation of Rabaul could now begin.
The March 1944 Japanese Offensive
By the start of 1944 General Hyakutake had finally been forced to admit that the landings at Empress Augusta Bay weren't a diversionary attack. He believed that the Americans had 20,000 combat troops and 10,000 ground crews within their beachhead, and decided to launch a large scale counterattack. This was to involve the 6th Division (General Kanda) and part of the 17th Division, a force of no more than 20,000 men. The Americans actually had twice as many men within the beachhead, as well as three functioning airfields. The Americal and 37th Divisions had around 27,000 men, and in total there were around 62,000 men within the American perimeter.
During the early months of 1944 the Japanese built roads across the island, and slowly moved their troops into position on hills inland of the American position. Artillery and heavier equipment was moved along the coast on landing barges and then dragged inland. These movements were reported by Allied scouts, coastwatchers and other sources, and copies of the Japanese plans were taken from dead soldiers. General Griswold was thus able to prepare for the Japanese offensive, denying them the advantage of surprise. The Japanese did have one advantage - the ground inland from the American positions continued to climb into the mountains, so the Japanese could see into the beachhead.
The Japanese force was split into three columns - Iwasa Unit, Magata Unit and Muda Unit, each of which was named after their commanding officer. Magata commanded on the right, Iwasa in the centre and Muda on the left. Magata and Iwasa would come up against the 37th Division and Muda the Americal Division.
General Hyakutake developed a complex plan that was designed to defeat the Americans in ten days. The attack was to begin on 8 March with an attack in the centre. Iwasa and Muda were then to unite and capture several key hills in the beachhead. On 11 March Magata would launch the main attack, which was designed to capture the two Piva airfields. The three Japanese columns would then turn towards the coast and advance towards the airfield at Cape Torokina, which was to fall on 17 March.
The Japanese artillery bombardment began at 5.45am on the morning of 8 March. It was heavier than the Americans had expected, but still not heavy enough to cause much damage to the American defences (the Americans were helped by the Japanese decision to focus on the airfields instead of the front lines). It did destroy one bomber and three fighters on the Piva airfields, and forced the American bombers to retreat to New Georgia, but it also attracted a powerful American counter-bombardment.
The Japanese attack finally began just after midnight on 8-9 March when two companies from the Japanese 23rd Infantry attacked up the steep slopes of Hill 700, defended by part of the US 145th Infantry. This first attack failed, but a second attack a couple of hours later did success in reaching the top of the hill. The Japanese had captured a small part of the American defensive perimeter and now held seven American pillboxes on Hill 700. During 9 March the Americans launched an attack on the new Japanese positions, and eventually managed to recapture five of the pillboxes, but the Japanese were able to fire into the beachhead, temporarily blocking one supply road.
10 March saw the Americans reduce the size of the Japanese salient on Hill 700. That night Iwasa launched a full scale assault on the American line. Along most the line the 37th Division held, but the salient on Hill 700 was expanded. The Americans reduced its size on 11 March, and finally eliminated it by 3.30pm on 12 March. On 13 March Iwasa Unit pulled back from contact with the American lines.
On 10 March Muda began his attack on Hill 260, which was defended by part of the Americal Division. This defensive position was just outside the main American perimeter, and on the night of 9-10 March small numbers of Japanese troops managed to get between the hill and the main lines. Early on 10 March the Japanese overwhelmed the main American defensive positions on the hill and forced the survivors of the small garrison to retreat from the fortified South Knob to North Knob. General Griswold ordered an immediate counterattack. Advancing troops linked up with the defenders of North Knob, and a two-pronged attack began before noon. This first attack failed, as did two more during the day.
On 11 March the reinforced Japanese force on Hill 260 launched an attack, but they were repulsed. The Americans also struggled to make any progress, and at the end of the day the situation was largely unchanged, with North Knob in American hands and South Knob held by the Japanese. American attacks on 12 March and 13 March also failed. On 14 March the Americans decided not to carry out any more direct attacks on the hill, but instead to slowly squeeze the Japanese out of their positions. On the following day the Japanese decided to move most of Muda's men from Hill 260 to reinforce Magata Force, which had made little progress. Hill 260 was eventually abandoned on 27 March at the start of the Japanese retreat.
On 17 March the Japanese decided to abandon their current attacks on the American line and instead prepare for a single concentrated attack against the 129th Infantry. A small force was left on Hill 260 but the rest of the attack force joined Magata Unit. Once again captured documents meant that the Americans learnt of the new Japanese plan.
After dark on 23 March the Japanese began an artillery bombardment of the American lines, followed by a general attack by most of the remaining troops. The Americans responded with heavy artillery and mortar fire, and most of the advancing Japanese units were stopped. There was one breakthrough, and the Japanese captured four pillboxes. Soon after dawn on 24 March the Americans launched a counterattack and recaptured all of the lost ground.
On 27 March the Japanese began a general retreat back towards their starting positions in the north and south of Bougainville. The failed attack had cost them 5,000 dead and 3,000 wounded, while the Americans had only lost 263 dead.
During April the Americans expanded their perimeter and also pursued the retreating Japanese. Some heavy fighting took place during this period, but the retreat continued. Some skirmishes were fought in May, before the two sides separated.
The failure of the March 1944 Japanese offensive ended the main fighting during the American period on Bougainville. A large bridgehead had been secured, three airfields had been built, and Rabaul was now within range of land based fighters. Something of an unofficial truce now broke out on the island, at least until the Americans were replaced by an Australian corps late in 1944. The Australians had the task of clearing Bougainville, and their campaign lasted from late in 1944 until the last Japanese troops on the island surrendered in August 1945.