'Operation 81' was a Japanese reinforcement convoy in the South-West Pacific theatre leading to the Battle of the Bismarck Sea (28 February/4 March 1943).
'Operation 81' was otherwise known as the 'Lae Resupply Convoy' under the command of Rear Admiral Masatomi Kimura. The convoy departed Rabaul in New Britain for Lae in New Guinea with a number of Imperial Japanese army transport vessels: these were the 2,746 Aiyo Maru, 2,319-ton Keizan Maru, 954-ton Kembu Maru loaded with fuel in drums, 6,493-ton Oigawa Maru, 3,793-ton Shinai Maru, 2,883-ton Taimei Maru and 6,801-ton Teiyo Maru carrying 1,923 soldiers, as well as the 4,500-ton collier Nojima Maru carrying 600 special naval landing force troops and desperately needed provisions. The transports were supplemented by a number of warships to provide protection and carry some 6,900 soldiers of Major General Hidemitsu Nakano’s 51st Division: these warships were the destroyers Shirayuki (flagship),Arashio, Asashio, Tokitsukaze, Yukikaze, Uranami, Shikinami and Asagumo.
The convoy’s movement led to the Battle of the Bismarck Sea on 2/4 March as aircraft of the US 5th Air Force and the Royal Australian Air Force (RAAF) attacked the convoy.
'Operation 81' resulted from the decision of Imperial General Headquarters during December 1942 to reinforce their position in the South-West Pacific, and among other things was to involve the delivery of some 6,900 troops from Rabaul directly to Lae. The Japanese appreciated from the start that the plan was fraught with risk as Allied air power in the area was strong, but the decision to proceed was made as the troops would otherwise have to be landed far from Lae and thereafter to move through inhospitable swamp, mountain and jungle terrain without roads before to reach Lae. On 28 February 1943, the convoy (eight transport vessels and eight destroyers), covered by about 100 fighters, departed Simpson Harbour in Rabaul.
The Allies had detected the Japanese preparations for 'Operation 81', naval codebreakers in Melbourne and Washington having decrypted and translated messages indicating the convoy’s intended destination and date of arrival. By this time, the Allied air forces had developed new techniques which they believed would improve significantly their chances of success in air attack on ships. They detected and shadowed the convoy, which came under sustained attack on 2/4 March by aircraft, and on 4 March by PT-boats and aircraft. All eight transports and four of the escorting destroyers were sunk: of the 6,900 embarked troops, only some 1,200 reached Lae, while of the other 5,700 about 2,700 were rescued by destroyers and submarines and returned to Rabaul, and the remaining 3,000 were either killed or drowned. This was a major reverse on New Guinea for the Japanese, who made no further attempts to reinforce Lae by ship, thus hindering to a marked degree the their efforts to halt Allied offensives in New Guinea, beginning with the 'Postern' offensive to retake Lae and the north coast of the Huon Gulf.
Six months after the Japanese 'Ai' attack on Pearl Harbor in December 1941, the USA had scored a huge strategic victory at the Battle of Midway in June 1942 as the Japanese attempted 'Mi' (ii). Swiftly seizing the strategic initiative, the USA landed forces on Guadalcanal in the south-eastern part of the Solomon islands group in 'Watchtower' during August 1942 at the start of a long campaign. The battle for Guadalcanal ended in Allied victory as the Japanese implemented the 'Ke' (ii) evacuation of their surviving troops from the island in early February 1943. At the same time, Australian and US forces in New Guinea repelled the Japanese land offensive along the Kokoda Trail. Going on the offensive, the Allied forces captured Buna and Gona in Papua, to the south-east of Lae and Salamaua, in the process eliminating the Japanese forces in that area.
The real object of the Allied counter-offensives in New Guinea and the Solomon islands group was to take the primary Japanese base at Rabaul on New Britain in a series of moves within the 'Cartwheel' plan, and clear the way for the eventual reconquest of the Philippine islands group after a strategic advance to the north. Recognising the threat, the Japanese continued to send land, sea and air reinforcements to the area in order to check the Allied advances.
In assessing the course of the battle for Guadalcanal and the campaign for Buna and Gona during December 1942, the Japanese came to appreciate the prospect that neither could be held. Imperial General Headquarters thus decided to reinforce the Japanese position in the South-West Pacific Area by despatching 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, commander of the 8th Area Army based at Rabaul, ordered Lieutenant General Hatazo Adachi’s 18th Army to secure Madang, Wewak and Tuluvu in New Guinea. On 29 December, Adachi ordered the 102nd Regiment and other units under the command of Major General Toru Okabe, the commander of the 51st Division's infantry group, to move from Rabaul to Lae and then to advance inland to take and hold Wau. After deciding to evacuate Guadalcanal on 4 January, the Japanese reallocated priority from the Solomon islands group to New Guinea, and opted to send the 20th Division and 41st Division to Wewak.
On 5 January 1943, the convoy of five transports and five destroyers delivering Okabe’s force, departed Rabaul for Lae. On the basis of 'Ultra' intelligence, USAAF and Royal Australian Air Force aircraft shadowed and then attacked the convoy, which was shielded by low cloud and Japanese fighter. The Allies claimed the destruction of 9 Japanese aircraft for the loss of 10 of their own.An Australian Consolidated PBY Catalina sank the transport Nichiryu Maru, and although destroyers rescued 739 of the 1,100 troops on board, the ship sank with all of Okabe’s medical and other supplies. Another transport, Myoko Maru, was damaged so severely at Lae by US North American B-25 Mitchell medium bombers that she had to be run onto the beach. Even so, the convoy succeeded in reaching Lae on 7 January and landing its troops, but Okabe was later defeated at Wau.
Most of the 20th Division came ashore at Wewak from naval transport vessels on 19 January, and the majority of the 41st Division followed on 12 February. Imamura and Vice Admiral Gunichi Mikawa, commander of the South-East Area Fleet, developed a plan to move the command post of the 18th Army's headquarters and the main body of the 51st Division from Rabaul to Lae on 3 March, with the rest of the 20th Division following to Madang on 10 March. The plan was deemed to be of high risk as Allied air power in the area was strong. The 18th Army's staff held war games that predicted losses of four out of 10 transports, and between 30 and 40 aircraft, which suggested that operation possessed only a 50% chance of success. However, were the troops to be landed at Madang, they faced a march of more than 140 miles (230 km) over inhospitable swamp, mountain and jungle terrain without roads. To boost the Japanese local air strength (three navy and two army fighter groups) assigned to protect the convoy, the Imperial Japanese navy temporarily detached 18 fighters from the aircraft carrier Zuiho's fighter group from Truk in the Carline islands group to Kavieng on New Ireland.
The Allies soon began to discover evidence of preparations for a new convoy, including the sighting on 7 February of a Japanese floatplane of the type normally used for anti-submarine patrols in advance of convoys. The commander of the Allied Air Forces South-West Pacific Area, Lieutenant General George C. Kenney, ordered an increase in the number of reconnaissance flights over Rabaul, and reconnaissance photograph taken on 14 February revealed the presence in the port of 79 vessels including 45 merchant ships and six transport vessels. It was now abundantly clear that preparations for another convoy were will advanced, but the destination of the convoy remained unclear. On 16 February, naval codebreakers completed the decryption and translation of a coded message revealing that the the Japanese intended to land convoys at Wewak, Madang and Lae. Codebreakers soon decrypted another message from the 11th Air Fleet to the effect that destroyers and six transports would reach Lae on 5 March or thereabouts. Another report indicated that the ships would reach Lae by 12 March. On 22 February, reconnaissance aircraft reported the presence of 59 merchant vessels in the harbour at Rabaul.
Kenney read this 'Ultra' intelligence in the office of General Douglas MacArthur, the Supreme Allied Commander, South-West Pacific Area, on 25 February. The prospect of an additional 6,900 Japanese troops in the Lae area disturbed MacArthur strongly, for the presence of these reinforcements might seriously affect his plans to capture and develop the area. Kenney ordered Brigadier General Ennis C. Whitehead, deputy commander of the 5th Air Force, and the commander of its advanced echelon in New Guinea. Under the 5th Air Force's somewhat atypical command arrangements, Whitehead controlled all Allied air units in New Guinea, including those of the RAAF grouped as Air Commodore Joe Hewitt’s No. 9 Operational Group. 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 trimmed, in the short term, to allow concentration of men and aircraft for a large attack on the convoy, and instructed Whitehead to move forward as many aircraft as possible. This was designed to ensure that Allied air strength could be as close as possible to the nearby captured airfields around Dobodura, where they would not be subject to the vagaries of weather in flights over the Owen Stanley mountain range. On 26 February, Kenney flew to Port Moresby, on the southern coast of Papua, to meet Whitehead. The two men inspected fighter and bomber units in the area, and agreed that the optimum location for the planned attack on the Japanese convoy was the Vitiaz Strait between New Guinea and New Britain. Kenney then returned to Brisbane on 28 February.
For Allied air power based in the South-West Pacific Area, any conventional strategic bombing campaign was impossible as industrial targets in Japan were well beyond the range of even the largest strategic bombers operating from bases in Australia and New Guinea. Thus the primary mission of the Allied bomber force in New Guinea 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 to sink a mere two ships sunk and damage three others. A change of tactics was clearly needed. Group Captain William 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.
In September 1942, Major Paul I. 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 0.5-in (12.7-mm) machine guns in their noses, and adding provision for two 450-US gal (1700-lire) fuel tanks to provide the aircraft with longer range. In December of the same year, an attempt was made to create a longer-ranged attack warplane by doing the same thing to a B-25 medium bomber to convert it into a 'commerce destroyer', tut this proved to be somewhat more difficult task. The resulting machine was nose-heavy despite the addition of lead ballast in the tail, and the vibration caused by the firing of the machine guns was great enough to cause rivets to pop out of the aeroplane’s skin. The tail guns and ventral turrets were removed, the latter being of little use in a low-flying type. The new tactic of having the B-25 for the strafing of ships was to be tried in the forthcoming air/sea battle.
The Allied air forces also adopted other innovative tactics. In February 1942, the RAAF had started to experiment with the concept of 'skip bombing', an anti-ship technique used by the British and Germans. Flying at very low altitude above the sea as they approached toward their targets, bombers would release their bombs which would then, ideally, ricochet across the surface of the water and explode against the side of the target ship, or under it or just over it. A similar technique was mast-high bombing, in which bombers approached the target at low altitude, in the order of 200 to 500 ft (60 to 150 m) at about 265 to 275 mph (425 to 445 km/h), and then drop to mast height of 10 to 15 ft (3 to 4.6) at a distance of about 600 yards (550 m) from the target. They then released their bombs at around 300 yards (270 m), 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. Practice missions were carried out against the wreck of the Pruth, a liner which had run aground in 1923.
The 5th Army 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 in the preceding six months, and availability was therefore low. The recently arrived 90th Bombardment Group was equipped with the Consolidated B-24 Liberator, but the aircraft were also suffering from maintenance problems. There were two medium groups: the 38th Bombardment Group equipped with the B-25 Mitchell, and the 22nd Bombardment Group equipped with the Martin B-26 Marauder. Two of the former’s four squadrons had been diverted to the South Pacific Area, however, and the latter had suffered so many losses that it had been withdrawn to Australia to be rebuilt.There was also one light group, the 3rd Attack Group, equipped with Douglas A-20 Havoc and B-25 Mitchell warplanes. The group was short of aircraft, and was furthermore critically short of aircrew. To offset the shortage of US crews, therefore, the USAAF had asked the RAAF for assistance. Australian aircrew were assigned to most of the group’s aircraft, serving in every role except aircraft commander.
In addition to the aircrew it had loaned to the USAAF, the RAAF had a number of units in the Port Moresby area. No. 30 Squadron, which had arrived in Port Moresby in September 1942, was equipped with the Bristol Beaufighter heavy fighter. Both the Beaufighter and No. 30 Squadron proved very capable in the low-level attack role. Also in the Port Moresby area were the 35th and 49th Fighter Groups, both equipped with Bell P-39, Curtiss P-40 Warhawk and Lockheed P-38 Lightning fighters, but only the last were suitable for the long-range escort role.
The eight transports and eight destroyers comprising the 'Operation 81' convoy, covered by about 100 fighters, departed Simpson Harbour in Rabaul on 28 February. During the January operation, the convoy’s course had hugged the southern coast of New Britain. This had made the provision of air cover an easy matter, but the fact of being close to airfields also made it possible for Allied warplanes to attack both the convoy and Japanese airfields at the same time. This time, therefore, the course adopted passed off the northern coast of New Britain in the hope that the Allies would be deceived into thinking that the convoy’s objective, straight to the west, 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 planned course would be particularly dangerous as the convoy would have to south through the restricted waters of the Vitiaz Strait, and then turn to the south-east and finally to the west to reach Lae to reach Lae from the east.
The transport ships were carrying 5,954 troops, and the destroyers another 958 troops. The ships were all combat-loaded to allow the speediest possible unloading at Lae. Adachi, commander of the 18th Army, travelled in the destroyer Tokitsukaze and Nakano, commander of the 51st Division, on the destroyer Yukikaze. Leading 'Operation 81, Kimura of the 3rd Destroyer Flotilla, flew his flag on the destroyer Shirayuki. All the ships carried troops, equipment and ammunition, the sole 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 evaded detection for some time as two tropical storms had struck the Solomon and Bismarck Seas between 27 February and 1 March, but at about 15.00 on 1 March the crew of a patrolling B-24 Liberator spotted the convoy. Eight B-17 Flying Fortresses were sent to the location but failed to locate the ships.
At dawn on 2 March, six RAAF A-20 Boston attack aircraft bombed Lae to reduce its ability to provide support. At about 10.00, another Liberator found the convoy. Eight B-17s were then despatched to deliver an attack on them, followed an hour later by another 20. The bombers located the convoy and attacked with 1,000-lb (455-kg) bombs from an altitude of 5,000 ft (1525 m). The bomber crews claimed to have sunk up to three transport ships, but the reality what that while the Kyokusei Maru had been sunk with 1,200 army troops, two other transports, the Teiyo Maru and Nojima Maru, had only been damaged. Eight Japanese fighters were destroyed and 13 damaged in the day’s action.
The destroyers Yukikaze and Asagumo rescued 950 of the Kyokusei Maru's survivors and then, being faster than the convoy, were detached to deliver the survivors at Lae. After completing this high-speed dash, the destroyers resumed their escort duties on the following day. Without the troop transport and two destroyers, the convoy came under renewed attack on the evening of 2 March by 11 B-17 bombers, which inflicted minor damage on a single transport. During the night, PBY Catalina flying boats of the RAAF’s No. 11 Squadron shadowed the convoy.
By 3 March, the convoy was within range of the air base at Milne Bay at the south-eastern tip of Papua, where eight Bristol Beaufort torpedo bombers of the RAAF’s No. 100 lifted. Adverse weather meant that only two of these 10 aircraft located the convoy, and neither scored any hits, but the weather cleared after the Japanese ships had rounded the Huon peninsula. At this point, 90 Allied aircraft took off from Port Moresby and directed their flight toward Cape Ward Hunt, while 22 A-20 Boston attack aircraft of the RAAF’s No. 22 Squadron attacked the Japanese fighter base at Lae, reducing the convoy’s air cover: such attacks continued through the day.
At 10.00, 13 B-17 bombers attacked the convoy and attacked from an altitude of 7,000 ft (2125 m). To make the attack more difficult, the Japanese ships started to manoeuvre, but this dispersed the convoy and reduced its ability to generate concentrated anti-aircraft fire. The American bombers attracted the attentions of Mitsubishi A6M Reisen 'Zero' fighters, which were in turn attacked by the bombers' P-38 Lightning escorts. One B-17 broke up in the air, its crew bailing out of the stricken machine. Japanese fighters machine-gunned the B-17’s hapless crewmen in the air and after they had come down in the water. Five of these Japanese fighters were swiftly tackled and shot down by three Lightning fighters, which were also lost. The Allied fighter pilots claimed the destruction of Zero fighters, while the B-17 crews claimed five more: the real Japanese fighter losses for the day were seven destroyed and three damaged.
B-25 medium bombers arrived shortly after this and attacked with 500-lb (227-kg) bombs dropped from altitudes 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 attacks at this time was only a few hits, but nonetheless left the convoy’s ships more widely separated and therefore more vulnerable to strafing and mast-head bombing attacks. Moreover, as the Japanese anti-aircraft fire was concentrated on the medium-altitude bombers. the opportunity for successful low-altitude attack was enhanced.
The 13 Beaufighter heavy fighters 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, and the Japanese ships turned toward them in the standard procedure to present torpedoes with smaller targets. This gave the Beaufighter fighters the opportunity to maximise the damage they inflicted on the ships' anti-aircraft guns, bridges and crews in strafing runs with their four 20-mm nose cannon and six 0.303-in (7.7-mm) wing machine-gun. Immediately after this, seven B-25 bombers of the 38th Bombardment Group’s 71st Bombardment Squadron bombed from about 2,450 ft (745 m) and another six such aircraft of the 405th Bombardment Squadron attacked at mast height.
The first Japanese ship to be hit was the Shirayuki, in this instance by a combination of strafing and bombing attacks. Almost all the men on the bridge were hit, these including Kimura, who was wounded. The detonation of one bomb triggered a magazine explosion which blew away the stern of the destroyer, which immediately started to settle in the water. The surviving members of the crew were transferred to Shikinami, and Shirayuki's hulk was scuttled. The destroyer Tokitsukaze was also hit and fatally damaged, her crew being taken off by Yukikaze. The destroyer Arashio was also hit, and them collided with the transport Nojima Maru, disabling the latter. Both the destroyer and the transport were then abandoned, and Nojima Maru being sunk in a later air attack.
Fourteen B-25 bombers returned during the afternoon of the same day, and claimed 17 hits or near misses. By this time, a third of the transports had been sunk or were sinking. As the Beaufighter fighters and B-25 bombers had expended their munitions, some USAAF A-20 Havoc warplanes of the 3rd Attack Group took over the assault. Another five hits were claimed by B-17 bombers of the 43rd Bombardment Group dropping their loads from higher altitudes. During the afternoon, further attacks from B-25 bombers of the USAAF and Boston bombers of the RAAF’s No. 22 Squadron.
All seven of the transport ships were hit, and most were burning or sinking in an area about 60 miles (100 km) to the south-east of Finschhafen, along with the destroyers Shirayuki, Tokitsukaze and Arashio. Four of the destroyers, namely Shikinami, Yukikaze, Uranami and Asagumo, recovered as many survivors as they could and then turned back toward Rabaul, accompanied by the destroyer Hatsuyuki, which had steamed from Rabaul to aid the stricken convoy and its escorts.
That night, 10 US Navy PT-boats, under the command of Lieutenant Commander Barry Atkins, departed to attack what was left of the convoy. Two of the 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 launched torpedoes and sank the crippled vessel. During the morning of 4 March, the destroyer Asashio was sunk when she was hit by a 500-lb (227-kg) bomb from a B-17 as she was rescuing [a]Arashio's survivors from Arashio. Of the four surviving destroyers, Yukikaze was undamaged.
Some 2,700 survivors were taken to Rabaul by the destroyers, and on 4 March another 1,000 or so survivors remained 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 on rafts and swimming or floating in the sea. This was later justified on the grounds that rescued men would have been landed at their military destination and returned to active service: some also justified this action as retaliation for the Japanese fighters attacks on survivors of the downed B-17. On 6 March, the Japanese submarines I-17 and I-26 rescued 170 survivors. Two days later, I-26 found another 54 men and landed them at Lae. Some hundreds of the survivors found their way to nearby islands. Some 18 survivors landed on Kiriwina island, 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 mounted a retaliatory raid on the airfield at Buna, which the Allies had captured in January, but the strafing fighters did little damage.
On Goodenough island, between 8 and 14 March 1943, patrols of the Australian 47th 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, one of these documents was discovered to be a copy of the Imperial Japanese Army List, with the names and postings of every officer, and thus provided a complete order of battle of the Imperial Japanese army, including many units which has hitherto been unreported. 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 against Japan.
The Battle of the Bismarck Sea was a disaster for the Japanese. Out of 6,900 troops so desperately 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; and about 2,890 Japanese soldiers and sailors were killed. The Allies lost 13 aircrew, 10 of them in combat the other three in an accident. There were also eight Allied wounded. Aircraft losses were one B-17 bomber and three P-38 fighters in combat, and one B-25 and one Beaufighter in accidents.
MacArthur issued a communiqué on 7 March, in this stating that 22 ships, including 12 transport vessels, three cruisers and seven destroyers had been sunk, and 12,792 troops killed. US Army Air Force Headquarters in Washington, DC, reconsidered the matter in the middle of 1943 and came to the conclusion that there were only 16 ships involved, but MacArthur’s South-West Pacific Area headquarters continued to insist that the original account was accurate.
The victory was a propaganda boon for the Allies, and one US newsreel claimed the Japanese had lost 22 ships, 15,000 troops and 102 warplanes.
The Allied air force command had used 233,847 rounds of ammunition, and dropped 261 500-lb (227-kg) and 253 1,000-lb (454-kg) bombs, claiming 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, some 48 were claimed as hits, 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 strafing runs by the Beaufighter warplanes had knocked out many of the ships' anti-aircraft defences. Aircraft attacking from multiple directions had confused and overwhelmed the Japanese defences, resulting in lower Allied casualties and more accurate bombing. The results therefore vindicated not just the tactics of mast-height attack, but of mounting co-ordinated attacks from multiple directions. The Japanese estimated that at least 29 bombs had hit a ship during the battle. This was a major improvement over the results of the Battle of Wau in January, when Allied aircraft attacked a Japanese convoy of five destroyers and five troop transports travelling from Rabaul to Lae, but managed to sink just one transport and cause the beaching of another.
Was is incontestable is the fact that the Japanese had suffered a significant defeat. Imamura’s chief-of-staff flew to Japan to report to Imperial General Headquarters, and one of the results was a decision to make no further attempt to land troops at Lae. Their losses in the Battle of the Bismarck Sea caused the Japanese to become very worried about the continued security of Lae and Rabaul. This resulted in an important change of strategy, and on 25 March a joint Army-Navy Central Agreement on South West Area Operations placed priority for operations in New Guinea over those in the Solomon islands group. the 18th Army was allocated larger quantities of shipping, ordnance and anti-aircraft units, which were sent to Wewak or Hansa Bay.
The planned movement of the 20th Division to Madang was revised in the light of the Battle of the Bismarck Sea: the formation’s redeployment was postponed for two days, and the destination was changed from Madang to Hansa Bay, farther to the west. In order to attempt a reduction in the Allied air threat, the 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 Curtiss P-40 fighter was lost in the air, but Allied fighters claimed to have shot down nine Japanese warplanes. The transport ships reached Hansa Bay unscathed on 12 March, and the troops then made their way south-east to Madang on foot or in barges. The 20th Division next became involved in an attempt to construct a road from Madang to Lae through the Ramu and Markham valleys. The division worked on the road for a few months, but its efforts were ultimately frustrated by New Guinea’s weather and the rugged terrain of the Finisterre mountain range.
A number of the Imperial Japanese navy’s submarines were made available for supply runs to Lae. These lacked the capacity to provide anything like the level of support the troops required, however, and an operation was effected 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 coast of New Guinea from Madang to Finschhafen, and along both the north and south coasts of New Britain to Finschhafen, and thence to Lae using army landing craft. It was by this means that the balance of the 51st Division finally made the trip to Lae in May. The necessity of delivering troops and supplies to the front in this manner gave the Japanese forces huge difficulty in their efforts to halt further Allied advances. Following the end of World War II, Japanese officers at Rabaul estimated that about 20,000 men died in transit from Rabaul to New Guinea, and this was a major factor in Japan’s defeat in the New Guinea campaign.
In April the commander-in-chief of the Combined Fleet, Admiral Isoroku Yamamoto, made use of the additional air resources allocated to Rabaul in the 'I' 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 became a casualty of Allied intelligence and air power in the Solomon islands group on 18 April 1943.