The 'Battle of the Bismarck Sea' was an air and naval engagement fought between US and Australian forces and Japanese forces in the South-West Pacific Area (2/4 March 1943).

In this battle, aircraft of the US 5^th Army Air Force and the Royal Australian Air Force attacked a Japanese convoy carrying troops to Lae in New Guinea, destroying most of the Japanese force and inflicting especially heavy ship and manpower losses on it. The convoy was a result of a Japanese decision of December 1942 by the Imperial General Headquarters decision to reinforce the Japanese position in the South-West Pacific Area. The resulting plan was created to redeploy some 6,900 troops from Rabaul, the Japanese base area on New Britain island, directly to Lae. The scheme was understood to be risky as Allied air power in the area was strong, but the decision to proceed was nonetheless taken as otherwise the troops would have to be landed a considerable distance from Lae and march through inhospitable swamp, mountain and jungle terrain without roads before reaching their destination. On 28 February 1943, the convoy of eight destroyers and eight troop transports, with an escort of about 100 single-engined fighters, departed from Simpson Harbour near Rabaul.

The Allies had detected the preparations for the convoy, and naval codebreakers in Melbourne and Washington, DC, had decrypted and translated messages indicating the convoy’s intended destination and date of arrival. By this period of the war, the Allied air forces had developed new techniques, such as skip bombing, which they hoped would improve the chances of successful air attack on ships. The Allies detected and shadowed the convoy, which came under sustained air attack on 2 and 3 March. Follow-on attacks by PT-boats and aircraft were made on 4 March on lifeboats and rafts. All eight transports and four of the escorting destroyers were sunk. Of the 6,900 troops so urgently required on New Guinea, only about 1,200 reached Lae. Another 2,700 were rescued by destroyers and submarines and returned to Rabaul. The Japanese made no further attempts to reinforce Lae by ship, greatly hindering their ultimately unsuccessful efforts to stop Allied offensives in New Guinea.

Six months after carrierborne aircraft of the Imperial Japanese navy attacked Pearl Harbor in 'Ai' on 7 December 1941, the USA won a monumental strategic victory in the 'Battle of Midway' during June 1942. Seizing the strategic initiative, the USA and its allies made the 'Watchtower' landing on Guadalcanal in the southern part of the Solomon islands group in August 1942, beginning a major land,sea and air campaign that lasted to February 1943. The Guadalcanal campaign ended in victory for the Allies with the withdrawal of Japanese forces from the island early in February 1943. At the same time, Australian and US forces in New Guinea repelled the Japanese land offensive along the Kokoda Track toward Port Moresby and, going on the offensive, captured Buna and Gona, destroying Japanese forces in that area.

The ultimate goal of the Allied counter-offensives in New Guinea and the Solomon islands group was to capture the main Japanese base at Rabaul on New Britain in what was later designated 'Cartwheel', and thus to clear the way for the eventual reconquest of the Philippine islands group. Recognising the threat, the Japanese continued to send land, naval and air reinforcements to the area in an attempt to check the Allied advances.

Reviewing the progress of the Guadalcanal campaign and the 'Battle of Buna-Gona' in December 1942, the Japanese faced the prospect that neither could be held. Accordingly, the Imperial General Headquarters decided to take steps to strengthen the Japanese position in the South-West Pacific Area by sending Lieutenant General Jusei Aoki’s 20^th Division from Korea to Guadalcanal and Lieutenant General Heisuke Abe’s 41^st Division from China to Rabaul. Lieutenant General Hitoshi Imamura, commander of the Japanese 8^th Area Army headquartered at Rabaul, ordered Lieutenant General Hatazo Adachi’s 18^th Army to secure Madang, Wewak and Tuluvu in New Guinea. On 29 December, Adachi ordered the 102^nd Regiment and other units under the command of Major General Toru Okabe, commander of the infantry group of Lieutenant General Hidemitsu Nakano’s 51^st Division, to move from Rabaul to Lae and advance inland to capture Wau. After deciding on 4 January to evacuate Guadalcanal, the Japanese switched their priorities from the Solomon islands group to New Guinea, and opted to send the 20^th Division and 41^st Division to Wewak.

On 5 January the convoy, which comprised five destroyers and five troop transports carrying Okabe’s force, departed Rabaul for Lae. Forewarned by 'Ultra', USAAF and Royal Australian Air Force aircraft located, shadowed and attacked the convoy, which was shielded by low clouds and Japanese fighters. The Allies claimed to have shot down 69 Japanese aircraft for the loss of 10 of their own machines. An RAAF Consolidated Catalina twin-engined flying boat sank the 5,447-ton transport Nichiryu Maru. Although destroyers rescued 739 of the 1,100 troops on board, the ship took to the bottom all of Okabe’s medical supplies. Another transport, the 4,103-ton Myoko Maru, was so severely damaged at Lae by USAAF North American B-25 Mitchell twin-engined medium bombers that she had to be beached. Nonetheless, the convoy succeeded in reaching Lae on 7 January and landing its troops. Even so, Okabe was defeated in the 'Battle of Wau'.

Most of the 20^th Division was landed at Wewak from high-speed naval transport vessels on 19 January, and most of the 41^st Division followed on 12 February. Imamura and Vice Admiral Gunichi Mikawa, commander of the South-East Area Fleet, now developed a plan to move the the headquarters of the 17^th Army and the main body of the 51^st Division from Rabaul to Lae on 3 March, followed by moving the remainder of the 20^th Division to Madang on 10 March. This plan was acknowledged to be risky as Allied air power in the area was strong. The 17^th Army's staff held war games that predicted losses of four out of 10 transports, and between 30 and 40 aircraft, and thus gave the undertaking only a 50% chance of success. On the other hand, if the troops were landed at Madang, they faced a march of more than 140 miles (225 km) over inhospitable swamp, mountain and jungle terrain without roads. To augment the area’s three naval and two army fighter groups assigned to the defence of the convoy, the Imperial Japanese navy temporarily detached 18 fighters of the fleet carrier Zuiho from Truk to Kavieng.

The Allies soon began to detect signs of the preparations for a new convoy operation. A Japanese floatplane of the type normally used for anti-submarine patrols in advance of convoys was sighted on 7 February. The commander of the Allied Air Forces South-West Pacific Area, Lieutenant General George C. Kenney, therefore ordered an increase in the number of reconnaissance sorties flown over Rabaul. On 14 February, aerial photographs showed 79 vessels in port, including 45 merchant ships and six transports. It was now abundantly clear that another convoy was being readied, but its destination was unknown. On 16 February, naval codebreakers in Melbourne and Washington, DC, finished decrypting and translating a message revealing the Japanese intention to land convoys at Wewak, Madang and Lae. Subsequently, codebreakers decrypted a message from the 11^th Air Fleet to the effect that destroyers and six transports would reach Lae on about 5 March. Another report indicated that they would reach Lae by 12 March. On 22 February, Allied reconnaissance aircraft reported 59 merchant vessels in Rabaul harbour.

Kenney read this 'Ultra' intelligence in the office of the Supreme Allied Commander, South-West Pacific Area, General Douglas MacArthur, on 25 February. The prospect of an additional 6,900 Japanese troops in the Lae area greatly disturbed MacArthur, as this reinforcement might seriously affect his plans to capture and develop the area. Kenney issued orders, delivered by courier, for Brigadier General Ennis C. Whitehead, the deputy commander of the 5^th Army Air Force, and the commander of its Advance Echelon in New Guinea. Under the 5^th AAF’s unusual command arrangements, Whitehead controlled the Allied air forces' units of all types in New Guinea, including RAAF units grouped as No. 9 Operational Group, under the command of Air Commodore J. E. Hewitt.

Kenney informed Whitehead of the proposed convoy date, and warned him about the usual Japanese pre-convoy air attack. He also urged that flying hours be reduced so as to allow for a large attack on the convoy, and instructed Whitehead to move forward as many aircraft as possible so that they could be close to the nearby captured airfields around Dobodura, where they would not be subject to the vagaries of the highly variable weather over the Owen Stanley mountain range. Kenney flew to Port Moresby on 26 February and met with Whitehead. The two generals inspected fighter and bomber units in the area, and agreed to attack the Japanese convoy in the Vitiaz Strait. Kenney returned to Brisbane on 28 February.

In the South-West Pacific Area, a conventional strategic bombing campaign was out of the question as industrial targets in Japan were well beyond the range of even the longest-ranged bombers operating from bases in Australia and New Guinea. Therefore, the primary mission of the Allied bomber force was interdiction of Japanese supply lines, especially the sea lanes. The results of the effort against the Japanese convoy in January, it had been ascertained, had been very disappointing: some 416 sorties had been flown with only two ships sunk and three damaged. Clearly, a change of tactics was in order, and Group Captain W. Garing of the RAAF and an officer on Kenney’s staff with considerable experience in air/sea operations, recommended that the Japanese convoys be subjected to simultaneous attack from different altitudes and directions. The Allied air forces adopted some innovative tactics. In February 1942, the RAAF began experimenting with skip bombing, an anti-shipping technique used by the British and Germans: flying only a few dozen feet above the sea toward their targets, bombers would release their bombs which would then, ideally, ricochet across the surface of the water and detonate on the side of the target ship, under it, or just over it. A similar technique was mast-height bombing, in which bombers would approach the target at low altitude, between 200 and 500 ft (60 and 150 m), at about 265 to 275 mph (426 to 443 km/h), and then drop down to mast height, 10 to 15 ft (3 to 4.6 m) at about 600 yards (550 m) from the target. They would then release their bombs at about 300 yards (270 m), aiming directly at the side of the ship. The 'Battle of the Bismarck Sea' would demonstrate that this latter was the more successful of the two tactics, which were not mutually exclusive: a bomber could drop two bombs, skipping the first and launching the second at mast height. In addition, as standard bomb fuses were designed to detonate immediately on impact, which would catch the attacking aircraft in the blast of its own bomb during low-altitude attacks, crews developed a delayed-action fuse. Practice missions were carried out against the wreck of the Pruth, a liner which had run aground in 1923. Six Allied aircraft crashed during these bombing and strafing practices.

In order for bombers to conduct skip or mast-height bombing, the target ship’s anti-aircraft guns would first have to be neutralised by strafing runs. For the latter task, Major P. I. Gunn and his men at the 81^st Depot Repair Squadron in Townsville, Queensland, modified some of the USAAF’s Douglas A-20 Havoc twin-engined light bombers by installing four 0.5-in (12.7-mm) machine guns in their noses during September 1942. Two 450-US gal (1700-litre) fuel tanks were added to provide the aircraft with greater range. An attempt was then made in December 1942 to create a longer-range attack aeroplane by doing the same thing to a B-25 medium bomber, but this proved to be somewhat more difficult. The resulting aeroplane was nose-heavy despite the addition of lead ballast in the tail, and the vibrations caused by firing the machine guns were enough to make rivets pop out of the aeroplane’s skin. The tail guns and belly turrets were removed, the latter being of little use if the aircraft was flying at low altitude. The new tactic of having the B-25 strafe ships would be tried in this battle.

The 5^th AAF had two heavy bomber groups. The 43^rd Bombardment Group was equipped with about 55 Boeing B-17 Flying Fortress four-engined aircraft, of which most had seen hard war service over the previous six months and therefore suffered from a low serviceability ate. The recently arrived 90^th Bombardment Group was equipped with Consolidated B-24 Liberator four-engined aircraft, but these too had maintenance problems. There were two medium groups: the 38^th Bombardment Group, equipped with the B-25, and the 22^nd Bombardment Group, equipped with the Martin B-26 Marauder twin-engined machine, but two of the former’s four squadrons had been diverted to the South Pacific Area, and the latter had taken so many losses that it had been withdrawn to Australia to be rebuilt. There was also one light group in the form of the 3^rd Attack Group, equipped with a mix of Douglas A-20 Havoc and B-25 Mitchell aircraft. This group was short of aircraft and also critically short of aircrew. To make up the numbers, the USAAF turned to the RAAF for help. Australian aircrew were assigned to most of the group’s aircraft, serving in every role except aircraft commander. In addition to the RAAF aircrew with the USAAF 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 twin-engined heavy fighter, and both the aircraft and the squadron proved adept at low-level attacks. Also in the Port Moresby area were the 35^th and 49^th Fighter Groups, both equipped with Bell P-39 Airacobra and Curtiss P-40 Warhawk single-engined machines and Lockheed P-38 Lightning twin-engined heavy fighters, but only the last were suitable for long-range escort missions.

Comprising the destroyers Arashio, Asashio, Asagumo, Shikinami, Shirayuki, Tokitsukaze, Uranami and Yukikaze, and the transports Aiyo Maru (2,746 tons), Kembu Maru (953 tons), Kyokusei Maru (3,794 tons), Oigawa Maru (6,494 tons), Sinai Maru (3,793 tons), Taimei Maru (2,883 tons), Teiyo Maru (6,870 tons) and Nojima Maru (8,215 tons), the convoy was escorted by about 100 fighters. The convoy assembled in and departed from Simpson Harbour on 28 February. During the January operation, the course which was adopted followed the southern coast of New Britain. This had made it easy 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. On this occasion, the route selected for what was known to the Japanese as 'Operation 81' lay along New Britain’s northern coast 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 overfly New Britain, allowing interdiction from Japanese air bases there, but the final leg of the voyage would be particularly dangerous as the convoy would have to negotiate the restricted waters of the Vitiaz Strait between New Britain and New Guinea.

The destroyers carried 958 troops while the transports took on board 5,954 men, and all the ships were combat-loaded to expedite unloading at Lae. The commander of the 18^th Army, Lieutenant General Hatazo Adachi, travelled on the destroyer Tokitsukaze, while the commander of the 51^st Division, Lieutenant General Hidemitsu Nakano was embarked on the destroyer Yukikaze. The escort’s commander was Rear Admiral Masatomi Kimura of the 3^rd Destroyer Flotilla and flew his flag on the destroyer Shirayuki. The warships escorted seven Imperial Japanese army transport vessels in the form of the Aiyo Maru, Kembu Maru, Kyokusei Maru, Oigawa Maru,Sinai Maru, Taimei Maru and Teiyo Maru, together with a single Imperial Japanese navy transport, the Nojima Maru. All but one of the ships carried troops, equipment and ammunition, the exception being the Kembu Maru, which carried 1,000 drums of aviation fuel and 650 drums of other fuel.

Steaming at 7 kt, the convoy was not detected by the Allies for some time, because of two tropical storms that struck the Solomon Sea and Bismarck Sea between 27 February and 1 March, but at about 15.00 on 1 March, the crew of a patrolling B-24 spotted the convoy. Eight B-17 bombers were despatched to the location but failed to locate the ships.

At dawn on 2 March, a force of six RAAF A-20 bombers attacked Lae to reduce its ability to provide support. At about 10.00, another Liberator found the convoy. Eight B-17 aircraft took off to attack the ships, followed an hour later by another 20. It was planned that the B-17 machines would rendezvous with P-38 fighters of the 9^th Fighter Squadron, but the B-17 bombers arrived early and faced Japanese fighters on their own for the initial air battle until the P-38 fighters arrived. The bombers found the convoy and attacked with 1,000-lb (454-kg) bombs from 5,000 ft (1525 m), and claimed to have sunk up to three transport ships: Kyokusei Maru had indeed been sunk carrying 1,200 army troops, but two other transport ships, the Teiyo Maru and Nojima Maru, were only damaged. Eight Japanese fighters were destroyed and 13 damaged in the day’s action, while nine B-17 bombers were damaged.

The destroyers Yukikaze and Asagumo rescued 950 survivors of the Kyokusei Maru from the water. Being faster than the convoy since its speed was dictated by that the slower transports, these two destroyers then detached from the group to disembark the survivors at Lae before resuming their escort duties on the following day. Without the troop transport and two destroyers, the convoy then came under attack once again on the evening of 2 March by 11 B-17 bombers, but suffered minor damage to only one transport. During the night, Catalina flying boats of the RAAF’s No. 11 Squadron took over the task of shadowing the convoy.

By 3 March, the convoy was within range of aircraft operating from the base at Milne Bay, from which eight Bristol Beaufort twin-engined torpedo bombers of the RAAF’s No. 100 Squadron took off.As a result of adverse weather, only two of the Beaufort aircraft found the convoy, and neither scored a hit, but the weather cleared after the Japanese ships had rounded the Huon peninsula. A force of 90 Allied aircraft took off from Port Moresby, and headed for Cape Ward Hunt, while 22 A-20 machines of the RAAF’s No. 22 Squadron 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 bombed from medium altitude of 7,000 ft (2135 m), causing the ships to manoeuvre, which dispersed the convoy formation and reduced the concentration of its anti-aircraft firepower. The B-17 bombers attracted Mitsubishi A6M Reisen 'Zero' single-engined fighters, which were in turn attacked by the P-38 escorts. One B-17 broke up in the air, and its crew took to their parachutes, some of the men then being machine-gunned by the Japanese fighters in the air or in the water after they had come down. Five of the Japanese fighters strafing the B-17 aircrew were promptly engaged and shot down by three P-38 fighters, which were also lost. The Allied fighter pilots claimed the destruction of 15 A6M fighters, and the B-17 crews claimed five more: the actual Japanese fighter losses for the day were seven destroyed and three damaged. B-25 bombers arrived shortly after this and released their 500-lb (227-kg) bombs at altitudes of between 3,000 and 6,000 ft (915 and 1830 m), reportedly causing two Japanese vessels to collide. The result of the B-17 and B-25 sorties was a few hits, but left the convoy’s ships separated and more vulnerable to strafers and mast-head bombers, and with the Japanese anti-aircraft fire focused on the medium-altitude bombers, this left an opening for minimum-altitude attacks.

The 13 Beaufighter warplanes of the RAAF’s No. 30 Squadron approached the convoy at low level to give the impression they were Beaufort machines making a torpedo attack. The ships turned to face them, which was the standard procedure to present a smaller target to torpedo bombers, allowing the Beaufighter aircraft to maximise the damage they inflicted on the ships' anti-aircraft guns, bridges and crews in strafing runs with their four 20-mm nose-mounted cannon and six 0.303-in (7.7-mm) wing-mounted machine guns. Immediately afterward, seven B-25 bombers of the 38^th Bombardment Group’s 71^st Bombardment Squadron bombed from about 2,460 ft (750 m), while six from the 405^th Bombardment Squadron attacked at mast height.

According to the official RAAF release on the Beaufighter attack, 'enemy crews were slain beside their guns, deck cargo burst into flame, superstructures toppled and burned'.

Shirayuki was the first ship to be hit, in this instance by a combination of 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 begin to sink. The crew was transferred to Shikinami, and Shirayuki was then scuttled. Tokitsukaze was also hit and fatally damaged, and her crew was taken off by Yukikaze. Arashio was hit, and collided with the transport Nojima Maru, disabling her. Both the destroyer and the transport were abandoned, and Nojima Maru was later sunk by an air attack.

A force of 14 B-25 bombers returned in the afternoon, claiming 17 hits or near misses. By this time, a third of the transport vessels had been sunk or were sinking. As the Beaufighter and B-25 warplanes had expended their munitions, some USAAF A-20 Havoc light bombers of the 3^rd Attack Group joined the fray. Another five hits were claimed by B-17 bombers of the 43^rd Bombardment Group from higher altitudes. During the afternoon, further attacks from USAAF B-25 bombers and Boston bombers of the RAAF’s No. 22 Squadron followed.

All seven of the surviving transport vessels were hit and most were burning or sinking about 60 miles (100 km) to the south-east of Finschhafen, along with the destroyers Shirayuki, Tokitsukaze and Arashio. The destroyers Shikinami, Yukikaze, Uranami and Asagumo recovered as many survivors as possible and then retired to Rabaul, accompanied by the destroyer Hatsuyuki, which had arrived from Rabaul to assist. That night, a force of 10 US Navy PT-boats, under the command of Lieutenant Commander B. Atkins, set out to attack the convoy. Two boats struck submerged debris and were forced to turn back. but the other eight arrived off Lae in the early hours of 4 March. Atkins spotted a fire that turned out to be the transport Oigawa Maru. PT-143 and PT-150 fired torpedoes at the crippled ship, sinking her. In the morning, the destroyer Asashio was sunk when a B-17 hit her with a 500-lb (227-kg) bomb while she was picking up survivors from Arashio. Only one destroyer, Yukikaze, was undamaged among the four surviving destroyers.

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 warplanes attacked Japanese rescue vessels, as well as the survivors from the sunken ships 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, but it was also retaliation for the Japanese fighters attacks on survivors of the downed B-17 bomber. On 6 March, the Japanese submarines I-17 and I-26 picked up 170 survivors. Two days later, I-26 found another 54 survivors and put them ashore at Lae. Hundreds of the Japanese survivors made their way to various islands: one group of 18 survivors landed on Kiriwina, where they were captured by the crew of PT-114, and another group made its way to Guadalcanal, only to be killed by a US patrol.

On 4 March the Japanese launched a retaliatory raid on the Buna airfield, the site of a base that the Allies had captured in January, although the fighters did little damage. Kenney later wrote that the Japanese reprisal occurred 'after the horse had been stolen from the barn. It was a good thing that the [Japanese] air commander was stupid. Those hundred airplanes would have made our job awfully hard if they had taken part in the big fight over the convoy on March 3^rd.'

On Goodenough island, between 8 and 14 March, Australian patrols of the 47^th 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 of the Imperial Japanese army. It therefore provided a complete order of battle of the Imperial Japanese army, including many units that had never before been reported. A mention of any Japanese officer could now be correlated with his unit. Copies were made available to intelligence units in every theatre of war.

The 'Battle of the Bismarck Sea' had been a disaster for the Japanese. Out of the 6,900 troops so badly needed in New Guinea, only about 1,200 reached Lae. Another 2,700 were saved by destroyers and submarines and returned to Rabaul. Thus something in the order of 2,890 Japanese soldiers and seamen had been killed. The Allies lost 13 aircrew, 10 of them in combat and three in an accident; there were also eight wounded. Aircraft losses were one B-17 and three P-38 machines in combat, and one B-25 and one Beaufighter in accidents. On 7 March, MacArthur issued a communiqué stating that 22 ships, including 12 transports, three cruisers and seven destroyers, had been sunk along with 12,792 troops. Army Air Force Headquarters in Washington, DC, looked into the matter in the middle of 1943 and concluded that there were only 16 ships involved, but the South-West Pacific Area’s headquarters considered the original account accurate. The victory was a propaganda boon for the Allies, with one US newsreel claiming the Japanese had lost 22 ships, 15,000 troops, and 102 aircraft. The New York Times, on its front page of 4 March cited the loss by the Japanese of 22 ships, 15,000 troops and 55 aircraft.

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. They claimed 19 hits and 42 near misses with the former and 59 hits and 39 near misses with the latter. Of the 137 bombs dropped in low-level attacks, 48 were claimed to have hit but only 29 of the 387 bombs dropped from medium altitude were claimed as hits. It was noted that the high- and medium-altitude attacks scored few hits but dispersed the convoy, while the Beaufighters' strafing runs had knocked out many of the ships' anti-aircraft defences. Aircraft attacking from several directions at once had confused and overwhelmed the Japanese defences, resulting in lower casualties and more accurate bombing. The results therefore vindicated: this was 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, and this was a major improvement over the hit rate in the 'Battle of Wau' back in January, when Allied aircraft had managed to sink just one transport and cause another to beach.

Imamura’s chief-of-staff was flown to Imperial General Headquarters to report on the disaster. It was decided that there would be no further attempts to land troops at Lae. The losses incurred in the 'Battle of the Bismarck Sea' caused grave concern for the security of Lae and Rabaul, and resulted in a change of strategy. On 25 March a joint army and navy agreement on operations in the South-West Pacific Area gave operations in New Guinea priority over those in the Solomon islands campaign. The 18^th Army was allocated additional shipping, ordnance and anti-aircraft units, which were sent to Wewak or Hansa Bay.

The planned deployment of the 20^th Division to Madang was revised in the light of the 'Battle of the Bismarck Sea'. The undertaking was postponed for two days, and the destination was altered from Madang to Hansa Bay farther to the west. To reduce the air threat, the Allied airfield at Wau was bombed 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 Allied fighters claimed nine Japanese aircraft. The transports reached Hansa Bay unscathed on 12 March and the troops made their way down to Madang on foot or in barges. The 20^th Division then became involved in an attempt to construct a road from Madang to Lae through the Ramu and Markham river valleys. The men of the division laboured on the road for the next few months, but their efforts were ultimately frustrated by New Guinea’s weather and the rugged terrain of the Finisterre mountain range.

Some submarines were made available for supply runs to Lae, but these lacked the capacity to provide full support the troops. An operation was carried out on 29 March in which four destroyers delivered 800 troops to Finschhafen, but the growing threat from Allied aircraft led to the development of routes along the New Guinea coast from Madang to Finschhafen and along the northern and southern coasts of New Britain to Finschhafen, thence to Lae using army landing craft. It was by this means that the remainder of the 51^st Division finally made the trip to Lae in May. The necessity of delivering troops and supplies to the front in this manner caused immense difficulties for the Japanese in their attempts to halt further Allied advances. After the war, Japanese officers at Rabaul estimated that around 20,000 troops were lost in transit from Rabaul to New Guinea, a significant factor in Japan’s ultimate defeat in the New Guinea campaign.

In April, Admiral Isoroku Yamamoto used the additional air resources allocated to Rabaul in 'I', an 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 and Yamamoto himself fell victim to Allied intelligence and air power in the Solomon islands group on 18 April 1943.