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, as part of Operation 81. 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 theatre. 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 the Japanese 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. On 28 February 1943, the convoy – comprising eight destroyers and eight troop transports with an escort of about 100 fighter aircraft – set out from Simpson Harbour in Rabaul.
The Allies had detected preparations for the convoy, and naval codebreakers in Melbourne (FRUMEL) and Washington, DC, had decrypted and translated messages indicating the convoy's intended destination and date of arrival. The Allied Air Forces had developed new techniques, such as skip bombing, that they hoped would improve the chances of a 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 on lifeboats and rafts. 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 unsuccessful efforts to stop Allied offensives in New Guinea.
Background
Allied offensives
Six months after Imperial Japan attacked Pearl Harbor in December 1941, the United States won a strategic victory at the Battle of Midway in June 1942. Seizing the strategic initiative, the United States and its Allies landed on Guadalcanal in the southern Solomon Islands in August 1942, beginning the Solomon Islands Campaign. The battle for Guadalcanal ended in victory for the Allies with the withdrawal of Japanese forces from the island in early February 1943. At the same time, Australian and American forces in New Guinea repelled the Japanese land offensive along the Kokoda Track. Going on the offensive, the Allied forces captured Buna–Gona, destroyed Japanese forces in that area.
The ultimate goal of the Allied counteroffensives in New Guinea and the Solomons was to capture the main Japanese base at Rabaul on New Britain, later codified as Operation Cartwheel, and clear the way for the eventual reconquest of the Philippines. Recognizing the threat, the Japanese continued to send land, naval, and aerial reinforcements to the area in an attempt to check the Allied advances.
Japanese plans
Reviewing the progress of the Battle of Guadalcanal and the Battle of Buna–Gona in December 1942, the Japanese faced the prospect that neither could be held. Accordingly, Imperial General Headquarters decided to take steps to strengthen the Japanese position in the SWPA by sending 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, the commander of the Japanese Eighth Area Army at Rabaul, ordered Lieutenant General Hatazō Adachi's XVIII Army to secure Madang, Wewak, and Tuluvu in New Guinea. On 29 December, Adachi ordered the 102nd Infantry Regiment and other units under the command of Major General Tōru Okabe, the commander of the infantry group of the 51st Division, to move from Rabaul to Lae and advance inland to capture Wau. After deciding to evacuate Guadalcanal on 4 January, the Japanese switched priorities from the Solomon Islands to New Guinea, and opted to send the 20th and 41st Divisions to Wewak.
On 5 January 1943, the convoy, which consisted of five destroyers and five troop transports carrying Okabe's force, set out for Lae from Rabaul. Forewarned by Ultra, United States Army Air Forces (USAAF) and Royal Australian Air Force (RAAF) aircraft spotted, 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. An RAAF Consolidated PBY Catalina sank the transport Nichiryu Maru. Although destroyers rescued 739 of the 1,100 troops on board, the ship took with it all of Okabe's medical supplies. Another transport, Myoko Maru, was so badly damaged at Lae by USAAF North American B-25 Mitchells 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 naval high-speed transports on 19 January 1943. The bulk of the 41st Division followed on 12 February. Imamura and Vice Admiral Gunichi Mikawa, the commander of the South East Area Fleet, developed a plan to move the command post of the headquarters of the Japanese XVIII Army and the main body of the 51st Division from Rabaul to Lae on 3 March, followed by moving the remainder of the 20th Division to Madang on 10 March. This plan was acknowledged to be risky because Allied air power in the area was strong. The XVIII Army staff held war games that predicted losses of four out of ten transports, and between 30 and 40 aircraft. They gave the operation only a 50–50 chance of success. If the troops were landed at Madang, though, they faced a march of more than 140 mi (230 km) over inhospitable swamp, mountain, and jungle terrain without roads. To augment the three naval and two army fighter groups in the area assigned to protect the convoy, the Imperial Japanese Navy temporarily detached 18 fighters from the aircraft carrier Zuihō's fighter group from Truk to Kavieng.
Allied intelligence
The Allies soon began detecting signs of preparations for a new convoy. A Japanese floatplane of the type normally used for antisubmarine patrols in advance of convoys was sighted on 7 February 1943. The Allied Air Forces SWPA commander – Lieutenant General George Kenney – ordered an increase in reconnaissance patrols over Rabaul. On 14 February, aerial photographs were taken that showed 79 vessels in port, including 45 merchant ships and six transports. Clearly, another convoy was being prepared, but its destination was unknown. On 16 February, naval codebreakers in Melbourne (FRUMEL) and Washington, DC, finished decrypting and translating a coded message revealing the Japanese intention to land convoys at Wewak, Madang, and Lae. Subsequently, codebreakers decrypted a message from the Japanese 11th Air Fleet to the effect that destroyers and six transports would reach Lae about 5 March. Another report indicated that they 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 the office of the Supreme Allied Commander, SWPA – General Douglas MacArthur – on 25 February. The prospect of an additional 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 out orders, which were sent by courier, for Brigadier General Ennis Whitehead, the deputy commander of the Fifth Air Force, and the commander of its Advance Echelon (ADVON) in New Guinea. Under the Fifth Air Force's unusual command arrangements, Whitehead controlled the Allied Air Forces units of all types in New Guinea. This included the RAAF units there, which were grouped as No. 9 Operational Group RAAF, under the command of Air Commodore Joe Hewitt.
Kenney informed Whitehead of the proposed convoy date, and warned him about the usual Japanese preconvoy air attack. He also urged that flying hours be cut back, so as to allow for a large strike on the convoy, and instructed him 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 weather over the Owen Stanley Range. Kenney flew up to Port Moresby on 26 February, where he 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.
Allied tactics
In the SWPA, a conventional strategic bombing campaign was out of the question, as industrial targets in Japan were well beyond the range of even the largest strategic 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 were very disappointing; some 416 sorties had been flown with only two ships sunk and three damaged; clearly, a change of tactics was in order. Group Captain Bill Garing, an RAAF officer on Kenney's staff with considerable experience in air-sea operations, including a tour of duty in Europe, recommended that 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 antishipping 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 explode at 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, 200 to 500 feet (61 to 152 m), at about 265 to 275 miles per hour (426 to 443 km/h), and then drop down to mast height, 10 to 15 feet (3.0 to 4.6 m) at about 600 yards (550 m) from the target. They would release their bombs around 300 yards (270 m) high, aiming directly at the side of the ship. The Battle of the Bismarck Sea would demonstrate that this was the more successful of the two tactics. The two techniques were not mutually exclusive; a bomber could drop two bombs, skipping the first and launching the second at mast height. In addition, as regular bomb fuses were designed to detonate immediately on impact, which would catch the attacking aircraft in its own bomb blast at low-altitude attacks, crews developed a delayed-action fuse. Practice missions were carried out against the wreck of the SS Pruth, a liner that had run aground in 1923.
For bombers to conduct skip or mast-height bombing, the target ship's antiaircraft artillery would first have to be neutralized by strafing runs. For the latter task, Major Paul I. "Pappy" Gunn and his men at the 81st Depot Repair Squadron in Townsville, Queensland, modified some USAAF Douglas A-20 Havoc light bombers by installing four .50-inch (12.7 mm) machine guns in their noses in September 1942. Two 450-US-gallon (1,700 L; 370 imp gal) fuel tanks were added, giving the aircraft more range. An attempt was then made in December 1942 to create a longer-range attack aircraft by doing the same thing to a B-25 medium bomber to convert it to a "commerce destroyer", but this proved to be somewhat more difficult. The resulting aircraft was nose-heavy despite added lead ballast in the tail, and the vibrations caused by firing the machine guns were enough to make rivets pop out of the skin of the aircraft. The tail guns and belly turrets were removed, the latter being of little use if the aircraft were flying low. The new tactic of having the B-25 strafe ships would be tried in this battle.
The Fifth Air Force had two heavy bomber groups. The 43rd Bombardment Group was equipped with about 55 Boeing B-17 Flying Fortresses. Most of these had seen hard war service over the previous six months, and their availability rate was low. The recently arrived 90th Bombardment Group was equipped with Consolidated B-24 Liberators, but they, too, had maintenance problems. Of the two medium groups, the 38th Bombardment Group was equipped with B-25 Mitchells, and the 22nd Bombardment Group was equipped with Martin B-26 Marauders, 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.
Also, a light group, the 3rd Attack Group, was equipped with a mix of Douglas A-20 Havocs and B-25 Mitchells. This group not only was short of aircraft, but also was critically short of aircrew, as well. 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, RAAF units were in the Port Moresby area. No. 30 Squadron RAAF, which had arrived in Port Moresby in September 1942, was equipped with the Bristol Beaufighter. Both the aircraft and the squadron proved adept at low-level attacks. Also in the Port Moresby area, the 35th and 49th Fighter Groups were both equipped with Bell P-39, Curtiss P-40 Warhawk, and Lockheed P-38 Lightning fighters, but only the last were suitable for long-range escort missions.