Battle of the Bismarck Sea

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. 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.|alt=A propellor aircraft on display in a museum: The wing tips are folded up. 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. An RAAF Consolidated PBY Catalina sank the transport . Although destroyers rescued 739 of the 1,100 troops on board, the ship took with it all of Okabe's medical supplies. Another transport, , 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 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 '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. 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; , seen here in front of his P-38 Lightning: He was shot down by Japanese fighters during the Battle of the Bismarck Sea.|alt=Man standing in front of a twin-propeller aircraft 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. 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. For bombers to conduct skip or mast-height bombing, the target ship's antiaircraft artillery would first have to be neutralized by strafing runs. Two 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 ws 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. 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. ==Battle==

Convoy departure from Rabaul The Japanese convoy – comprising eight destroyers and eight troop transports with an escort of about 100 Mitsubishi A6M Zero fighters – assembled and departed from Simpson Harbour in Rabaul on 28 February. The A6Ms from the Japanese fighter base at Lae formed a combat air patrol over the convoy. The destroyers were fast and maneuverable, but with most of their weaponry focused on antisurface capabilities, their antiaircraft defenses were weak, as most only had two antiaircraft machine guns or cannons, while their 5-inch dual-purpose guns were not effective in an antiaircraft role.|alt=A ship under way, with splashes on both sides. The destroyers carried 958 troops, while the transports took 5,954. All the ships were combat loaded to expedite unloading at Lae. The commander of the Japanese XVIII Army, Lieutenant General Hatazō Adachi, travelled on the destroyer , while that of the 51st Division, Lieutenant General Hidemitsu Nakano, was on board the destroyer . The escort commander – Rear Admiral Masatomi Kimura of the 3rd Destroyer Flotilla – flew his flag from the destroyer . The other five destroyers were , , , and . They escorted seven Army transports: (2,716 gross register tons), (950 tons), (5,493 tons), (6,494 tons), (3,793 tons), (2,883 tons), and (6,870 tons). Rounding out the force was the lone Navy transport (8,125 tons). All the ships carried troops, equipment, and ammunition, except for the Kembu Maru, which carried 1,000 drums of avgas and 650 drums of other fuel. During the January operation, a course was followed that hugged the south coast of New Britain. This had made providing air cover easy, but being close to the airfields also made Allied air forces attacks on both the convoy and the airfields possible at the same time. This time, a route was chosen along the north 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 fly over New Britain, allowing 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 Vitiaz Strait. The Japanese named the convoy "Operation 81." First attacks The convoy, moving at , was not detected for some time, because of two tropical storms that struck the Solomon and Bismarck Seas between 27 February and 1 March, but around 15:00 on 1 March, the crew of a patrolling B-24 Liberator heavy bomber spotted the convoy. Eight B-17 Flying Fortresses were sent to the location, but failed to locate the ships. At dawn on 2 March, a force of six RAAF A-20 Bostons attacked Lae to reduce its ability to provide support. Around 10:00, another Liberator found the convoy. Eight B-17s took off to attack the ships, followed an hour later by another 20. The B-17s were planned to rendezvous with P-38 fighters from the 9th Fighter Squadron, but the B-17s arrived early and faced the Japanese fighters on their own for the initial air battle until the P-38s arrived. They found the convoy and attacked with bombs from . They claimed to have sunk up to three merchant ships. Kyokusei Maru had sunk, carrying 1,200 army troops, and two other transports, Teiyo Maru and Nojima, were damaged. Eight Japanese fighters were destroyed and 13 damaged in the day's action, while nine B-17s were damaged. The destroyers Yukikaze and Asagumo plucked 950 survivors of Kyokusei Maru from the water. These two destroyers, being faster than the convoy, since its speed was dictated by the slower transports, broke away from the group to disembark the survivors at Lae. The destroyers resumed their escort duties the next day. The ships turned to face them, the standard procedure to present a smaller target to torpedo bombers, allowing the Beaufighters to maximise the damage they inflicted on the ships' antiaircraft guns, bridges, and crews in strafing runs with their four nose cannons and six wing-mounted machine guns. Immediately afterward, seven B-25s of the 38th Bombardment Group's 71st Bombardment Squadron bombed from about , while six from the 405th 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". Fourteen B-25s returned that afternoon, reportedly claiming 17 hits or near misses. By this time, a third of the transports had been sunk or were sinking. As the Beaufighters and B-25s had expended their munitions, some USAAF A-20 Havocs of the 3rd Attack Group joined in. Another five hits were claimed by B-17s of the 43rd Bombardment Group from higher altitudes. During the afternoon, further attacks from USAAF B-25s and Bostons of No. 22 Squadron RAAF followed. All seven of the transports were hit and most were burning or sinking about southeast of Finschhafen, along with the destroyers Shirayuki, Tokitsukaze, and Arashio. Four of the destroyers – Shikinami, Yukikaze, Uranami, and Asagumo – picked up as many survivors as they could and then retired to Rabaul, accompanied by the destroyer , which had come from Rabaul to assist. Only one destroyer, Yukikaze, was undamaged among the four surviving destroyers. Some 2,700 survivors were taken to Rabaul by the destroyers. On 4 March, another 1,000 or so survivors were adrift on rafts. as well as being retaliation for the Japanese fighter planes attacking survivors of the downed B-17 bomber. On 4 March, the Japanese mounted a retaliatory raid on the Buna airfield, the site of a base that the Allies had captured back in January, though the fighters did little damage. Kenney wrote in his memoir that the Japanese reprisal occurred "after the horse had been stolen from the barn. It was a good thing that the Nip 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 3rd." ==Aftermath==

The battle was a disaster for the Japanese. Of 6,900 troops who were badly 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. About 2,890 Japanese soldiers and sailors were killed. The Allies lost 13 aircrew, 10 of whom were lost in combat, while three others died in an accident, with also eight wounded. Aircraft losses were one B-17 and three P-38s in combat, and one B-25 and one Beaufighter in accidents. MacArthur issued a communiqué on 7 March stating that 22 ships, including 12 transports, three cruisers, and seven destroyers, had been sunk along with 12,792 troops, based on a report by SWPA which also boasted exorbitant claims that included 60 enemy aircraft shot down, six destroyers or light cruisers sunk, and 11-14 merchantmen sunk. Army Air Force Headquarters in Washington, DC, looked into the matter in mid-1943 and concluded that only 16 ships were involved, but GHQ SWPA considered the original account accurate. The victory was a propaganda boon for the Allies, with one United States newsreel claiming the Japanese had lost 22 ships, 15,000 troops, and 102 aircraft. The New York Times, on its front page on March 4, 1943, cited the loss by the Japanese of 22 ships, 15,000 troops, and 55 aircraft, though the Japanese lost only 15 aircraft over the course of the raids. The Allied Air Forces had used 233,847 rounds of ammunition and dropped 261 500-pound and 253 1,000-pound bombs. They claimed 19 hits and 42 near misses with the former and 59 hits and 39 near misses from the latter. Of the 137 bombs dropped in low-level attacks, 48 (35%) were claimed to have hit but only 29 (7.5%) of the 387 bombs dropped from medium altitude. This compared favourably with efforts in August and September 1942, when only 3% of bombs dropped were claimed to have scored hits. Of note, the high- and medium-altitude attacks scored few hits, but dispersed the convoy, while the strafing runs from the Beaufighters had knocked out many of the ships' antiaircraft 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 not just the tactics of mast height attack, but also of mounting coordinated attacks from several directions. This was a big improvement over the Battle of Wau back in January, when Allied aircraft attacked a Japanese convoy consisting of five destroyers and five troop transports travelling from Rabaul to Lae, but managed to sink just one transport and beach another. The losses incurred in 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-Navy Central Agreement on South West Area Operations gave operations in New Guinea priority over those in the Solomon Islands campaign. The XVIII Army was allocated additional shipping, ordnance, and antiaircraft units, which were sent to Wewak or Hansa Bay. The transports reached Hansa Bay unscathed on 12 March and the troops made their way down to Madang on foot or in barges. The 20th Division then became involved in an attempt to construct a road from Madang to Lae through the Ramu and Markham Valleys. It toiled on the road for the next few months, but its efforts were ultimately frustrated by the New Guinea weather and the rugged terrain of the Finisterre Range. 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 to New Guinea from Rabaul, 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 Operation I-Go, an air offensive designed to redress the situation by destroying Allied ships and aircraft in New Guinea and the Solomon Islands. The operation was indecisive and Yamamoto became a casualty of Allied intelligence and air power in the Solomon Islands on 18 April 1943. ==Game theory==

In 1954, O. G. Haywood, Jr., wrote an article in the Journal of the Operations Research Society of America in which game theory was used to model the decision-making in the battle. Since then, the name of the battle has been applied to this particular type of two-person zero-sum game. As a very brief summary, the scenario modeled revolves around the decision Lt. General Kenney had to make just prior to the real-life battle. The Japanese commander had to choose one of two courses of action: Either sail his convoy north of the New Britain island, or south of it. General Kenney likewise had to choose: Focus any reconnaissance efforts north of the island, or south of it. This means four outcomes are possible, and game theory explains Gen. Kenney's real-life decision to concentrate his reconnaissance airplanes north of New Britain. ==Footnotes==