Operation No. 81

This was a Japanese reinforcement convoy operation leading to the Battle of the Bismarck Sea in General Douglas MacArthur’s South-West Pacific Area (28 February/4 March 1943).

During this air-against-sea battle, aircraft of Major General George C. Kenney’s US 5th AAF and Air Commodore Joseph E. Hewitt’s No. 9 Operational Group of the Royal Australian Air Force attacked and effectively destroyed a Japanese convoy carrying troops to Lae in New Guinea.

The Battle of the Bismarck Sea was fought between 2 and 4 March 1943 in the area of General Douglas MacArthur’s South-West Pacific Area, and took the form of air attacks by the US 5th AAF and the Royal Australian Air Force on the ‘Operation No. 81’ convoy carrying troops to Lae in North-East New Guinea. Most of the Japanese task force was destroyed, and Japanese suffered heavy troop losses.

The background to ‘Operation No. 81’ is to be found in the fact that six months after the Japanese ‘Ai’ attack on Pearl Harbor in December 1941, the USA won a major strategic victory in the Battle of Midway at the end of the Japanese ‘Mi’ operation. Now possessing the strategic initiative, the USA and allies launched the ‘Watchtower’ landing on Guadalcanal in the south-eastern part of the Solomon islands group on 1 August 1942, so starting the Solomon islands campaign. The battle for Guadalcanal ended in victory for the Allies with the ‘Ke’ (i) withdrawal of the surviving Japanese forces from the island early in the first nine days of February 1943. At the same time, Australian forces in North-East New Guinea and Papua repulsed the Japanese ‘Ri’ land offensive along the Kokoda Trail toward Port Moresby and the, going over to the offensive, captured the Japanese base area in the little settlements of Buna, Gona and Sanananda, destroying Japanese forces in that area.

The ultimate goal of the twin Allied counter-offensives in New Guinea and the Solomon islands group was the seizure of the primary Japanese base at Rabaul on New Britain in a strategic effort later codenamed as ‘Cartwheel’, and thus clear a major part of the way for the eventual reconquest of the Philippine islands group, which the Japanese had conquered in ‘M’ (i). Recognising the threat they now faced in the South-West Pacific Area, the Japanese continued to send land, sea and air reinforcements into the area in an effort to check the Allied advances.

Assessing the situation in the Battle of Guadalcanal and the Battle of Buna–Gona in December 1942, the Japanese came to the conclusion that they could not prevails in either of these, and must consider what they had to so in the aftermath of the loss of Guadalcanal and the area of Buna, Gona and Sanananda. Imperial General Headquarters therefore decided to start the reinforcement of the Japanese position in the South-West Pacific Area by despatching Lieutenant General Jusei Aoki’s 20th Division from Korea to Guadalcanal and Lieutenant General Heisuke Abe’s 41st Division from China to Rabaul. Lieutenant General Hitoshi Imamura, commanding the 8th Area Army at Rabaul, ordered Lieutenant General Hatazo Adachi’s 18th Army to secure Madang, Wewak and Tuluvu in North-East New Guinea, and as a result on 29 December Adachi ordered the 102nd Regiment and other units under the command of Major General Toru Okabe, commander of the 51st Division’s infantry group, to move from Rabaul to Lae and then advance inland to capture Wau in the Watut river valley.

Following the decision of 4 January to concede Guadalcanal and evacuate the remnants of Lieutenant General Harukichi Hyakutake’s 17th Army, the Japanese changed their operational priority in the area from the Solomon islands group to New Guinea, and it was decided to send Aoki’s 20th Division and Abe’s 41st Division to Wewak on the central part of North-East New Guinea’s north coast.

On 5 January a convoy of five troop transports carrying Okabe’s force, together with five destroyers, departed Rabaul for Lae. The Allies knew of the Japanese movement as a result of ‘Ultra’ decrypts and, despite the presence of low cloud and a fighter escort, the convoy was located, shadowed and attacked by US and Australian warplanes. The Allied aircraft claimed to have shot down 69 Japanese aircraft for the loss of 10 of their own, and an Australian Consolidated PBY Catalina flying boat sank the transport Nichiryu Maru: destroyers rescued 739 of the 1,100 troops on board, but the ship took down with it all of the Okabe force’s medical supplies. Another transport, Myoko Maru, was so badly damaged at Lae by USAAF North American B-25 Mitchell medium bombers that it had to be beached. Nonetheless, the convoy succeeded in reaching Lae on 7 January and landing its troops, but Okabe was defeated in the Battle of Wau.

Most of the 20th Division was landed at Wewak from high-speed naval transport vessels on 19 January, and most of the 41st Division followed on 12 February. Imamura and Vice Admiral Gunichi Mikawa, commander of the South-East Area Fleet, now drafted a plan to move the headquarters of the 17th Army and the main strength of the 51st Division from Rabaul to Lae on 3 March, with the remainder of the 20th Division transported to Madang on 10 March. From the start, it was acknowledged that the plan was risky because of the strength of Allied air power in the area. The 17th Army war-gamed the plan, and this predicted the loss of four of the 10 transport vessels as well as something between 30 and 40 aircraft, and in overall terms predicted only a 50% chance of success. However, if the troops were landed at Madang, they would then have to undertake a march of more than 140 miles (225 km) over roadless swamp, mountain and jungle terrain. To augment the local air strength of three navy and two army fighter groups assigned to the protection of the convoy, the Japanese navy air force temporarily detached from Truk in the Caroline islands group 18 Mitsubishi A6M Reisen ‘Zero’ fighters from the fleet carrier Zuiho’s fighter group, and these flew to Kavieng on the island of New Ireland.

The Allies quickly learned of the Japanese preparations for a new convoy undertaking. A Japanese floatplane of the type normally used for anti-submarine patrols in advance of convoys was sighted on 7 February 1943. Kenney, in his capacity as the Allied Air Forces, South-West Pacific Area, ordered a greater number of reconnaissance flights over Rabaul, and on 14 February aerial photographs revealed the presence of 79 vessels including 45 merchant ships and six transport vessels. It was clear that another convoy operation was being readied, but its destination was as yet unknown. On 16 February, naval codebreakers in Melbourne, Australia, and Washington, DC, finished decrypting and translating a message revealing that the Japanese intended to run convoys to Wewak, Madang and Lae, and the codebreakers later decrypted a message from Vice Admiral Jinichi Kusaka’s 11th Air Fleet that destroyers and six transport vessels would reach Lae on or about 5 March, while another report indicated that the ships would reach Lae by 12 March. On 22 February, reconnaissance aircraft reported 59 merchant vessels in the harbour at Rabaul.

Kenney read this ‘Ultra’ intelligence in MacArthur’s office on 25 February. The prospect of the arrival of a further 6,900 Japanese troops in the Lae area greatly disturbed MacArthur, as they might seriously affect his plans to capture and develop the area. Kenney wrote orders, which were sent by courier, to Brigadier General Ennis C. Whitehead, the 5th AAF’s deputy commander, and the commander of the 5th AAF’s Advance Echelon in New Guinea. Under the 5th AAF’s unusual command arrangements, Whitehead controlled all Allied Air Forces units in New Guinea, this including RAAF units, which were grouped as No. 9 Operational Group.

Kenney informed Whitehead of the convoy’s schedule and warned him about the usual Japanese pre-convoy air attack. He also urged that flying hours be curtailed to allow preparations for a maximum-strength air assault on the convoy, and instructed him to move as many aircraft as he could to locations as close as possible to the nearby captured airfields around Dobodura, where they would not be subject to the vagaries of weather over the mountains of the Owen Stanley range. Kenney himself flew to Port Moresby on 26 February to discuss the forthcoming effort with Whitehead. The two generals inspected fighter and bomber units in the area, and agreed to attack the Japanese convoy in the Vitiaz Strait between North-East New Guinea and the south-western tip of Nw Britain. Kenney returned to Brisbane on 28 February.

In the South-West Pacific Area there was no opportunity for a conventional strategic bombing campaign, as the industrial targets in the Japanese home islands lay well beyond the range of even the largest strategic bombers operating from bases in Australia and New Guinea. The primary mission of the Allied bomber force was thus the interdiction of the Japanese lines of communication, especially the sea lanes. The results of the effort against the Japanese convoy in January had proved to be disappointing, for the commitment of 416 sorties had led to the Japanese loss of only two ships sunk and three damaged. Clearly, a change of tactics was needed. Group Captain Bill Garing, an Australian officer on Kenney’s staff with considerable experience in air/sea operations, recommended that Japanese convoys be subjected to attack simultaneously from different altitudes and directions.

In September 1942, Major Paul I. Gunn and the men of the 81st Depot Repair Squadron in Townsville, Queensland, had modified some USAAF Douglas A-20 Havoc light attack bombers by the addition of four 0.5-in (12.7-mm) machine guns in their noses and the installation of two 450-US gal (1703-litre) fuel tanks to give the aircraft greater range. In December 1942, an attempt was then made to create a longer-range attack warplane by doing the same thing to a B-25 medium bomber, but this task proved more difficult. The resulting warplane was nose-heavy despite the addition of lead ballast in the tail, and the vibration caused by firing the machine guns was enough to make rivets pop out of the aeroplane’s skin. The tail guns and ventral turrets were removed, the latter being of little use when the aircraft were operating at low level. The new tactic of using the B-25 to strafe ships would be tried in the forthcoming battle.

The Allied Air Forces also adopted new tactics. In February 1942, the RAAF began to experiment with skip bombing, an anti-shipping technique used by the British and Germans. Flying a very low altitude above the sea toward their targets, the warplanes released bombs which then, ideally, ricocheted across the surface of the water to impact the side of the target ship and explode. A similar technique was mast-height bombing, in which the bomber approached the target at an altitude of between 200 to 500 feet (60 and 150 m), at about 235 kt (270 mph; 435 km/h), before descending to mast-head height at distance of about 600 yards (550 m) from the target and finally releasing its bombs at a distance of around 300 yards (275 m), aiming directly at the side of the ship. The Battle of the Bismarck Sea later demonstrated that this was the more successful of the two tactics. The two techniques were not mutually exclusive, however, for the bomber could drop two bombs, skipping the first and launching the second at mast-head height. Training missions were carried out against the wreck of Pruth, a liner which had run aground in 1923.

The 5th AAF possessed two heavy bomber groups. The 43rd Bombardment Group was equipped with about 55 Boeing B-17 Flying Fortress aircraft, most of which had seen hard service over the previous six months and therefore suffered from a low serviceability rate. The recently arrived 90th Bombardment Group was equipped with the Consolidated B-24 Liberator, but this too had maintenance problems. There were two medium bomber groups: the 38th Bombardment Group equipped with the B-25 Mitchell, and the 22nd Bombardment Group equipped with the Martin B-26 Marauder, but two of the former’s four squadrons had been diverted to the South Pacific Area, and the latter had suffered losses so great that it had been withdrawn to Australia to be rebuilt. There was also a light bomber group, the 3rd Attack Group, equipped with a mixture of A-20 Havoc and B-25 Mitchell aircraft. This last group was short of aircraft, and was also critically short of aircrew. To make up the numbers the Americans turned to the Australians for assistance. Australian aircrew were assigned to most of the group’s aircraft, serving in every role except aircraft commander.

In addition to the Australian aircrew with US squadrons, there were RAAF units in the Port Moresby area. No. 30 Squadron, which had arrived in Port Moresby in September 1942, was equipped with the Bristol Beaufighter heavy coastal fighter, and the Squadron and its aircraft proved adept in the low-level attack role. Also in the Port Moresby area were the 35th and 49th Fighter Groups, both equipped with Bell P-400, Curtiss P-40 Warhawk and Lockheed P-38 Lightning fighters, but only the last were suitable for long-range escort missions.

The Japanese convoy of eight transport vessels and eight destroyers, covered by some 100 fighters, departed Rabaul’s Simpson Harbour on 28 February. During the January operation the ships had shaped a course which hugged the south coast of New Britain while steaming to the south-west through the Solomon Sea. This had made it easy for the Japanese to provide air cover, but being close to the airfields also made it possible for the Allied Air Forces to attack both the convoy and the airfields at the same time. For ‘Operation No. 81’, the selected route followed the north coast of New Britain through the Bismarck Sea, in the hope that the Allies would be deceived into thinking that the convoy’s objective was Madang. Allied air attacks on the convoy at this point would have to fly over New Britain, allowing easy interdiction from Japanese air bases there, but the final leg of the voyage would be particularly dangerous, because the convoy would have to negotiate the restricted waters of the Vitiaz Strait.

The destroyers and transport vessels carried 958 and 5,954 troops respectively, and all of the ships were combat loaded to expedite the process of unloading at Lae. Adachi, commander of the 18th Army, travelled on the destroyer Tokitsukaze, Lieutenant General Hidemitsu Nakano, commander of the 51st Division, was on board the destroyer Yukikaze, and Rear Admiral Masatomi Kimura of the 3rd Destroyer Flotilla, flew his flag in the destroyer Shirayuki. The other five destroyers were Arashio, Asashio, Asagumo, Shikinami and Uranami. The eight destroyers escorted seven army transports, in the form of the 2,716-ton Aiyo Maru, 950-ton Kembu Maru, 5,493-ton Kyokusei Maru. 6,494-ton Oigawa Maru, 3,793-ton Sinai Maru, 2,883-ton Taimei Maru and 6,870-ton Teiyo Maru, as well as the 8,125-ton naval transport vessel Nojima. Seven of the ships carried troops, equipment and ammunition, but the little Kembu Maru carried 1,000 drums of aviation fuel and 650 drums of other fuel.

The 7-kt convoy remained undetected for several days because of two tropical storms that struck the Solomon Sea to the south of New Britain and the Bismarck Sea to the north of this island in the period between 27 February and 1 March, but at about 15.00 on 1 March the crew of a patrolling B-24 heavy bomber spotted the convoy. Eight B-17 bombers were despatched to the reported location but failed to locate the convoy.

At dawn on 2 March, six Australian A-20 light bombers attacked Lae to reduce its ability to provide support, and at about 10.00 on the same day another Liberator relocated the convoy. Eight B-17 bombers, followed after 60 minutes by another 20, were launched to attack the ships, found the convoy and attacked with 1,000-lb (454-kg) bombs dropped from and altitude of 5,000 ft (1525 m). The Flying Fortress bombers claimed to have sunk up to three merchant ships, but while Kyokusei Maru had in fact been sunk carrying with her 1,200 troops, two other transports, Teiyo Maru and Nojima, had only been damaged. Eight Japanese fighters were destroyed and 13 damaged in the day’s action.

The destroyers Yukikaze and Asagumo recovered from the water some 950 survivors of Kyokusei Maru and then departed the convoy to land the survivors at Lae before returning on the following day. The ships remaining in the convoy were attacked again on the evening of 2 March by 11 B-17 bombers, which inflicted minor damage to only one transport. During the night, Catalina flying boats of the RAAF’s No. 11 Squadron assumed the task of shadowing the convoy.

By 3 March the convoy was within range of the air base at Milne Bay, on the south-eastern tip of Papua, and eight Bristol Beaufort torpedo bombers from the RAAF’s No. 100 Squadron took off from there. The weather was poor, and only two of the aircraft located the convoy, neither scoring a hit, but the weather cleared after the convoy rounded the Huon peninsula and gradually shaped its course to the east in the direction of Lae. A force of 90 Allied aircraft lifted off from Port Moresby, and headed to the north-east in the direction of Cape Ward Hunt, about two-thirds of the way from Milne Bay to Lae, while 22 A-20 aircraft of the RAAF’s No. 22 Squadron RAAF attacked the Japanese fighter base at Lae, reducing the convoy’s air cover. Attacks on the base continued throughout the day.

At 10.00, 13 B-17 bombers reached the convoy and attacked from an altitude of 7,000 ft (2135 m). This caused the ships to manoeuvre, dispersing the convoy’s tight formation and reducing its ability to concentrate its anti-aircraft firepower. The B-17 attack attracted a number of ‘Zero’ fighters, which were in turn attacked by the P-38 escorts. One B-17 broke up in the air, its crew taking to its parachutes: Japanese fighters machine gunned some of the B-17 crew members as they descended and attacked others after they had entered the water. Five of the Japanese fighters strafing the B-17 aircrew were promptly engaged and shot down by three Lightnings, which were also lost. The Allied fighter pilots claimed 15 Zero fighters destroyed, while the B-17 crews claimed five more.However, the actual Japanese fighter losses for the day were seven destroyed and three damaged. B-25 bombers arrived shortly after this and delivered 500-lb (227-kg) bombs at altitudes between 3,000 and 6,000 ft (915 and 1830 m), reportedly causing two Japanese vessels to collide. The B-17 and B-25 sorties scored few hits but left the convoy’s ships farther separated from each other, rendering them more vulnerable to strafing aircraft and bombers attacking at mast-head height, and the Japanese concentration of their anti-aircraft fire against the medium-altitude bombers left an opening for low-altitude attack.

The 13 Beaufighter warplanes of No. 30 Squadron approached the convoy at low level to give the impression they were Beaufort aircraft intending to deliver a torpedo attack. The ships turned to face them, which was the standard procedure designed to offer torpedo bombers a smaller target, allowing the Beaufighter warplanes to maximise the damage they inflicted on the ships’ anti-aircraft guns, bridges and crews in strafing runs with their forward-firing armament of four 20-mm cannon and six 0.303-in (7.7-mm) machine guns. Immediately after this, seven B-25 bombers of the 38th Bombardment Group’s 71st Bombardment Squadron bombed from about 2,500 ft (760 m), while six aircraft of the 405th Bombardment Squadron attacked at mast-head height.

The first ship to be hit was Shirayuki, which came under strafing and bombing attacks. Almost all the men on the bridge became casualties, including Kimura, who was wounded. One bomb hit started a magazine explosion that caused the stern to break off and the ship to sink. The surviving men of her crew were transferred to Shikinami, and Shirayuki’s hulk was scuttled. The destroyer Tokitsukaze was also hit and fatally damaged. Her crew was taken off by Yukikaze. The destroyer Arashio was hit, and collided with the transport Nojima, disabling her. Both the destroyer and the transport were abandoned, and Nojima was later sunk by air attack.

Fourteen B-25 bombers returned during the afternoon of the same day, and claimed 17 hits or near misses. By this time, a third of the transports were sunk or sinking. As the Beaufighter and B-25 warplane had expended their munitions, some A-20 attack aircraft of the 3rd Attack Group joined the fray. Another five hits were claimed by B-17 bombers of the 43rd Bombardment Group attacking from higher altitude. During the afternoon, further attacks from US B-25 and Australian A-20 warplanes, the latter of No. 22 Squadron, followed.

All seven of the transports were hit, and most were burning or sinking in a location about 60 miles (100 km) to the south-east of Finschhafen, along with the destroyers Arashio, Shirayuki and Tokitsukaze. The destroyers Asagumo, Shikinami, Uranami and Yukikaze recovered as many survivors as they could and then turned back toward Rabaul, accompanied by the destroyer Hatsuyuki, which had been despatched from Rabaul to assist. During the night which followed, a force of 10 US PT-boats under the command of Lieutenant Commander Barry Atkins departed to attack the remnants of the ‘Operation No. 81’ convoy. Two boats struck submerged debris, however, and were thus compelled to turn back. The other eight PT-boats arrived off Lae in the early hours of 4 March, and Atkins spotted a fire that turned out to be the transport Oigawa Maru: PT-143 and PT-150 fired torpedoes which sank the crippled vessel. In the morning which followed, a fourth destroyer, Asashio, was sunk when a B-17 hit her with a 500-lb (227-kg) bomb as she was recovering some of Arashio’s survivors.

Some 2,700 survivors were taken to Rabaul by the destroyers, and on 4 March another 1,000 or so survivors were adrift on rafts. During the evenings of 3, 4 and 5 March, PT-boats and aircraft attacked Japanese rescue vessels, as well as the survivors from the sunken vessels on life rafts and swimming or floating in the sea. This was later justified on the grounds that rescued servicemen would have been rapidly landed at their military destination and promptly returned to active service, and was also retaliation for the attacks of Japanese fighters on survivors of the downed B-17 bomber.

On 6 March, the Japanese submarines I-17 and I-26 collected 170 survivors. Two days later, I-26 found another 54 and put them ashore at Lae. Hundreds of other survivors made their way to various islands. One band of 18 landed on Kiriwina island, in the Trobriand islands group, where they were captured by PT-114. Another group made its way to Guadalcanal, only to be killed by a US patrol.

On 4 March the Japanese undertook a retaliatory fighter raid on Buna airfield, the site of a base which the Allies had captured in January, but did little damage.

On Goodenough island, between 8 and 14 March 1943, patrols of the Australian 47th Battalion found and killed 72 Japanese, captured 42 and found another nine dead on a raft. One patrol killed eight Japanese who had landed in two flat-bottomed boats, in which were found some documents in sealed tins. On translation by the Allied Translator and Interpreter Section, one document turned out to be a copy of the Japanese Army List, with the names and postings of every officer in the Japanese army, and thus provided a complete order of battle of the Japanese army, including many units that had never before been reported. A mention of any Japanese officer could now be correlated with his unit, and copies of the translation were made available to intelligence units in every theatre of war against Japan.

The battle was a major Japanese defeat. Of the 6,900 troops so sorely needed in New Guinea, only about 1,200 made it to Lae, another 2,700 were saved by destroyers and submarines and returned to Rabaul, and about 2,890 Japanese soldiers and sailors were killed. The Allies lost 13 aircrew, 10 of them in combat and three others in an accident; there were also eight wounded. Allied aircraft losses were one B-17 and three P-38 machines in combat, and one B-25 and one Beaufighter in accidents. MacArthur miscounted, and on 7 March claimed that 22 ships, including 12 transports, three cruisers and seven destroyers, had been sunk, and 12,792 troops killed. USAAF headquarters in Washington, DC, looked into the matter in the middle of 1943 and arrived at the conclusion that there were only 16 ships involved, but the South-West Pacific Area nonetheless adhered to its original story.

The Allied Air Forces had expended 233,847 rounds of ammunition, and dropped 261 500-lb (227-kg) and 253 1,000-lb (454-kg) bombs, in the process claiming 19 hits and 42 near misses with the lighter bombs, and 59 hits and 39 near misses with the heavier bombs. Of the 137 bombs dropped in low-level attacks, 35% were claimed as hits, but only 7.5% of the 387 bombs dropped from medium altitude. This compared very favourably with operations in August and September 1942, when only 3% of bombs dropped were claimed to have scored hits. It was noted that the high- and medium-altitude attacks scored few hits but did serve to disperse the convoy, while the strafing runs from the Beaufighter heavy fighters had knocked out many of the ships’ anti-aircraft defences. Simultaneous attacks from several directions had confused and overwhelmed the Japanese defences, resulting in lower casualties and more accurate bombing. The results therefore vindicated not just the tactics of mast-height attack, but of mounting co-ordinated attacks from several directions.

The Japanese estimated that at least 29 bombs had hit a ship during the battle, which represented a major improvement over the hit rate in Battle of Wau during January, when Allied aircraft attacked a Japanese convoy of five destroyers and five troop transports travelling from Rabaul to Lae but managed to sink just one transport and another to beach itself.

Right from the date of the Battle of the Bismarck Sea, there was no doubt that the Japanese had suffered a major defeat. Major General Rimpei Kato, the 8th Area Army’s chief-of-staff flew to Japan to report directly to Imperial General Headquarters, which decided that there would be no more attempts to land troops at Lae.

The losses incurred in ‘Operation No. 81’ led to severe worries for the security of Lae and Rabaul, and paved the way to a change of strategy. On 25 March an Army-Navy Central Agreement on South-West Area Operations allocated operations in New Guinea priority over those in the Solomon islands group. The 18th Army was given additional shipping, ordnance and anti-aircraft units, which were sent to Wewak or Hansa Bay. The planned movement of the 20th Division to Madang was revised in the light of recent events: the operation was postponed for two days, and the destination was altered from Madang to Hansa Bay farther to the west. To reduce the threat of Allied air power, the Japanese bombed the airfield at Wau on 9 March, and that at Dobodura on 11 March. Three Allied aircraft were destroyed on the ground, and one P-40 was lost in the air, but the Allied fighters claimed to have shot down nine Japanese warplanes. The transports reached Hansa Bay unscathed on 12 March, and the troops moved to Madang on foot or in barges. The 20th Division was then committed to an attempt to build a road from Madang to Lae through the Ramu and Markham river valleys. The formation worked on the road for the next few months, but its efforts were ultimately frustrated by New Guinea’s weather and the rugged nature of the mountains of the Finisterre range.

Some submarines were made available for supply runs to Lae, but these boats lacked the payload capacity to serve as the sole support for the troops there. On 29 March four destroyers delivered 800 troops to Finschhafen, but the ever-growing threat of Allied air power led to the development of army landing craft routes along the coast of the Huon peninsula from Madang to Finschhafen, and along both the north and south coasts of New Britain and then across the Dampier and Vitiaz Straits to Finschhafen, and thence to Lae: it was this would allowed the remainder of the 51st Division to reach Lae in May. The movement of troops and supplies in this manner was logistically difficult and caused the Japanese great difficulties in their attempts to halt further Allied advances. After the war, Japanese officers at Rabaul estimated that some 20,000 men were lost in transit from Rabaul to New Guinea, and that this was a major factor in Japan’s defeat in the New Guinea campaign.

In April, Admiral Isoroku Yamamoto, commander of the Combined Fleet, used the additional air resources allocated to Rabaul for the ‘I’ air offensive designed to redress the situation by destroying Allied ships and aircraft in New Guinea and the Solomon islands group. The operation was indecisive.