The battle of the Bismarck Sea (2-4 March 1943) saw repeated Allied air attacks almost totally destroy a Japanese convoy attempting to get reinforcements from Rabaul to the bases at Lae and Salamaua on the north-east coast of New Guinea. A total of twelve of the sixteen Japanese ships involved in the attempt were sunk, and fewer than 1,000 men reached their destination.
The Japanese were already aware of the dangers posed by Allied air power. In January 1943 they had attempted to send the 102nd Infantry Regiment from Rabaul to Lae to join with the Naval troops already in the area. This movement had been detected by the Allies and in a series of air attacks they sank two transports. Although three quarters of the regiment reached Lae safely, they had lost half of their supplies.
The regiment, commanded by Major General Toru Okabe, was then used in an attempt to capture the Australian outpost at Wau (28-30 January 1943). This attack was also defeated with the help of Allied air power, which allowed reinforcements to be flown into the besieged base. Okabe suffered heavy losses in the attack, and requested reinforcements.
The Japanese commanders at Rabaul believed that Lae and Salamaua were an essential part of their defensive perimeter, and so they decided to send 6,900 men from the 51st Division to reinforce the area. A lack of alternatives meant that the division would have to travel by sea in a fleet of eight transport ships and eight destroyers. They also had around two hundred aircraft within range. The division's equipment was spread evenly between the eight transports so that the loss of a single ship wouldn't be a disaster.
The Allies had more aircraft in the area, with 207 bombers and 129 fighters based in Papua, but they lacked anti-shipping weapons. General Kenney's Fifth Air Force was aware of this weakness, and had practiced 'skip-bombing', a technique similar to the famous 'bouncing bomb'. 500lb bombs would be dropped at very low altitude and hopefully would hit the vulnerable sides of the transport ships. According to the official US Army history of the campaign Kenney had picked up the idea of 'skip-bombing' from the RAF. Kenney's men had also modified some of their B-25s to carry extra .50in forward firing guns that they could use on strafing attacks.
The Japanese convoy left Rabaul at midnight on 28 February. Allied reconnaissance aircraft spotted them on 1 March. They were found again early on 2 March, by which point they were near Cape Gloucester, on the north-western corner of New Britain.
This put them outside medium bomber range, but well within the range of Kenney's heavy B-17 Flying Fortresses and B-24 Liberators. Supported by long range P-38 Lightings the heavy bombers attacked the convoy. Heavy bombers were often ineffective against ships, but on this occasion they managed to sink one (the Kyokusei Maru) and damage two of the eight transports. 950 men from the sunken ship were rescued by the destroyers. On the night of 2-3 March two destroyers rushed these men to Lae, although without their equipment.
By 3 March the Japanese were within range of the medium bombers. The first attack of the day was carried out by Australian Beauforts carrying torpedoes (Torbeaus), but these didn’t score any hits.
The second attack was on a larger scale. Thirteen Beaufighters, thirteen B-17s, thirteen B-24s and twelve B-25s took part in the attack. The Beaufighters suppressed the Japanese anti-aircraft fire, while the heavy bombers attacked from medium altitude. Finally the B-25s made their low level attack. Japanese aircraft attempted to intervene, but the Allies only lost three P-38s and one B-17. During this attack the special service vessel Nohima (one of the transport ships) sank, the destroyers Arashio, Shirayuki and Tokitsukaze took mortal damage. The other transport ships were all either sunk or left sinking.
A third attack was launched on the afternoon of 3 March. There were now five destroyers left afloat (at this point the Arashio was still afloat). The Arashio was sunk during this attack, but the other destroyers survived. They managed to rescue just under 5,000 of the troops, but they had to abandon the attempt to get them to Lae, and instead took them back to Rabaul and Kavieng. Around 3,000 men were lost in the battle, and only the 950 taken by destroyers ever reached Lae.
The last transport ship was sunk by PT boats on the night of 3/4 March.
The battle of the Bismarck Sea was a crushing defeat for the Japanese. The much-needed reinforcements failed to reach Lae. The Japanese abandoned any future attempt to use transport ships in the Bismarck Sea, and the few reinforcements that did reach Lae and Salamaua had to come by submarine or small boats operating carefully at night. Wartime reports of the battle were unusually confused. Because the two forces hadn't been in constant contact the Allies weren't sure how many ships had been in the convoy, or if new ships had joined later on. Many reports thus gave the Japanese up to twelve transports, and added cruisers to the escort force.