The Battle of the Bismarck Sea (2–4 March 1943) took place in the South West Pacific Area (SWPA) during World War II when aircraft of the U.S. Fifth Air Force and the Royal Australian Air Force (RAAF) attacked a Japanese convoy carrying troops to Lae, New Guinea. Most of the Japanese task force was destroyed, and Japanese troop losses were heavy.
The Japanese convoy was a result of a Japanese Imperial General Headquarters decision in December 1942 to reinforce their position in the South West Pacific. A plan was devised to move some 6,900 troops from Rabaul directly to Lae. The plan was understood to be risky, because Allied air power in the area was strong, but it was decided to proceed because otherwise the troops would have to be landed a considerable distance away and march through inhospitable swamp, mountain and jungle terrain without roads before reaching their destination.
The Allies had detected preparations for the convoy, and naval codebreakers in Melbourne (FRUMEL) and Washington, D.C., had decrypted and translated messages indicating the convoy's intended destination and date of arrival. The Allied Air Forces had developed new techniques they hoped would improve the chances of successful air attack on ships. They detected and shadowed the convoy, which came under sustained air attack on 2–3 March 1943.
Follow-up attacks by PT boats and aircraft were made on 4 March. All eight transports and four of the escorting destroyers were sunk. Of 6,900 troops who were badly needed in New Guinea, only about 1,200 made it to 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.