Series of WWII battles
The Admiralty Islands campaign (Operation Brewer) was a series of battles in the New Guinea campaign of World War II in which the United States Army's 1st Cavalry Division took the Japanese-held Admiralty Islands.
Acting on reports from airmen that there were no signs of enemy activity and the islands might have been evacuated, General Douglas MacArthur accelerated his timetable for capturing the Admiralties and ordered an immediate reconnaissance in force. The campaign began on 29 February 1944 when a force landed on Los Negros, the third-largest island in the group. By using a small, isolated beach where the Japanese had not anticipated an assault, the force achieved tactical surprise, but the islands proved to be far from unoccupied. A furious battle over the islands ensued.
In the end, air superiority and command of the sea allowed the Allies to heavily reinforce their position on Los Negros. The 1st Cavalry Division could then overrun the islands. The campaign officially ended on 18 May 1944. The Allied victory completed the isolation of the major Japanese base at Rabaul that was the ultimate objective of the Allied campaigns of 1942 and 1943. A major air and naval base was developed in the Admiralty Islands that became an important launching point for the campaigns of 1944 in the Pacific. This campaign marked the end of MacArthur's Operation Cartwheel, which was a multi-theater operation conducted to turn the powerful Japanese base of Rabaul into a de facto prisoner-of-war camp.
Background.
Geography.
The Admiralty Islands lie north east of the mainland of New Guinea and west of Rabaul, only two degrees south of the equator. The climate is tropical, with constant high temperatures and high humidity and an annual rainfall of . Thunderstorms are common. December to May is the north west monsoon season, with prevailing winds from that direction.
The largest island in the group is Manus Island, which is about across from east to west and wide from north to south. The interior is mountainous, with peaks rising to and largely covered with thick tropical rainforest. The largely uncharted coastline had numerous reefs. The shoreline consisted mostly of mangrove swamp. 
The third largest island in the province, Los Negros lies to the northeast of Manus, from which it is separated by the narrow Loniu Passage. The island contains two important harbours of its own, Papitalai on the west coast, which connects with Seeadler Harbour, and Hyane on the east coast. The two are separated by a wide sandy spit. Here, the natives built a skidway over which they could drag canoes between the two harbours. Los Negros curves horseshoe-like, forming a natural breakwater for Seeadler Harbour, the remainder of which is enclosed by Manus and a series of smaller islands. The main entrance was through a wide passage between Hauwei and Ndrilo Islands. Seeadler Harbour is about across from east to west and wide from north to south, and up to deep.
Allied plans.
In July 1942, the Joint Chiefs of Staff approved a series of operations against the Japanese bastion at Rabaul, which blocked any Allied advance along the northern coast of New Guinea towards the Philippines or northward towards the main Japanese naval base at Truk. In keeping with the overall Allied grand strategy of Europe first, the immediate aim of these operations was not the defeat of Japan but merely the reduction of the threat posed by Japanese aircraft and warships based at Rabaul to air and sea communications between the United States and Australia. By agreement among the Allied nations, in March 1942 the Pacific theatre was divided into the South West Pacific Area, under General Douglas MacArthur, and the Pacific Ocean Areas, under Admiral Chester W. Nimitz. Rabaul fell within MacArthur's area but the initial operations in the southern Solomon Islands came under Nimitz. The Japanese reaction was more violent than anticipated and some months passed before the Guadalcanal Campaign was brought to a successful conclusion. Meanwhile, General MacArthur's forces—primarily Australian—fought off a series of Japanese offensives in Papua in the Kokoda Track Campaign, Battle of Milne Bay, Battle of Buna-Gona, and the Battle of Wau.
At the Pacific Military Conference in March 1943, the Joint Chiefs of Staff approved the latest version of General MacArthur's Elkton plan for an advance on Rabaul. Owing to a shortage of resources, particularly heavy bomber aircraft, the final stage of the plan, the capture of Rabaul itself, was postponed until 1944. By July 1943, the Joint Chiefs were considering the possibility of neutralising and bypassing Rabaul, but the navy would still need a forward fleet base. The Admiralty Islands, already a part of the Elkton plan, could serve this purpose, as they contained flat areas for airstrips, space for military installations, and Seeadler Harbour, which was large enough to accommodate a naval task force. On 6 August 1943, the Joint Chiefs of Staff adopted a plan that called for the neutralisation rather than the capture of Rabaul, and scheduled the invasion of the Admiralty Islands for 1 June 1944.
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Task Force Brewer Assault Echelon Units
Brigadier General William C. Chase
Throughout January 1944, AirSols aircraft based in the Solomon Islands and Royal Australian Air Force (RAAF) aircraft based on Kiriwina kept up a sustained air offensive against Rabaul. Under steady and relentless pressure, the Japanese air defence began to weaken, allowing a landing to be made on 15 February by New Zealand troops on the Green Islands, which lie little more than from Rabaul. On 16 and 17 February, the US Pacific Fleet's Task Force 58 attacked the main Japanese base at Truk. Most Japanese aircraft were recalled to defend Truk and 19 February saw the last significant interception of Allied aircraft over Rabaul. Meanwhile, on 13 February General MacArthur, who received an intelligence windfall from the capture of Japanese Army code books by his Australian soldiers at Sio, had issued orders for the invasion of the Admiralty Islands, codenamed Operation Brewer, which was now scheduled for 1 April. Forces assigned included the 1st Cavalry Division; No. 73 Wing RAAF, providing close air support; the 592nd Engineer Boat and Shore Regiment (EBSR); the US Marines' 1st Amphibious Tractor Battalion; and US Naval Construction Battalions ("Seabees") to build the naval base—a total of 45,000 personnel. However, on 23 February 1944 three Fifth Air Force B-25 Mitchell bombers flew low over Los Negros. The airmen reported that there were no signs of enemy activity and the islands had been evacuated. Lieutenant General George Kenney, the commander of Allied Air Forces in the South West Pacific Area, went to MacArthur and proposed that the unoccupied islands be quickly taken by a small force. According to Kenney: "The General listened for a while, paced back and forth as I kept talking, nodded occasionally, then suddenly stopped and said: That will put the cork in the bottle."
Orders went out on 24 February 1944 for a reinforced squadron of the 1st Cavalry Division to carry out a reconnaissance in force in just five days time. If the Admiralty Islands were indeed evacuated, they would be occupied and a base developed. If the enemy was unexpectedly strong, then the force could be withdrawn. General MacArthur and Vice Admiral Thomas C. Kinkaid, the commander of Allied Naval Forces in the South West Pacific Area, would be on hand to make the decision but otherwise they delegated command to Rear Admiral William Fechteler, the commander of Amphibious Group 8 of Rear Admiral Daniel E. Barbey's VII Amphibious Force. To accommodate them, the light cruiser was ordered to sea. At the time, she was in Brisbane, with over 300 of her crew on shore leave. Trucks with bull horns broadcast the code word recalling the crew. To achieve surprise, and to reach the Admiralty Islands in just five days, high speed transports (APDs) were required; Landing Ships, Tank (LSTs) were too slow to make the required distance in the time. Only three APDs were available: , and . Each could accommodate 170 men. The remaining troops were carried on nine destroyers: , , , , , , , and . Between them, the destroyers and APDs carried 1,026 troops.
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Task Force Brewer Supporting Echelon Units
Colonel Hugh Hoffman
This force was commanded by Brigadier General William C. Chase, commander of the 1st Brigade, 1st Cavalry Division. It included the three rifle troops and the heavy weapons troop of the 2nd Squadron, 5th Cavalry; a platoon from Battery B, 99th Field Artillery Battalion with two 75 mm pack howitzers; the 673rd Anti-Aircraft Machine Gun Battery (Airborne); and 29 Australians of the Australian New Guinea Administrative Unit (ANGAU), who were to assist in gathering intelligence and dealing with the native population, some 13,000 of whom lived in the islands. Once the decision to remain was known, a follow-up force with the rest of the 5th Cavalry and 99th Field Artillery Battalion, 40th Naval Construction Battalion, and 2,500 measurement tons of stores would depart from Finschhafen in six Landing Ships, Tank (LSTs), each towing an LCM of Company E, 592nd EBSR. When an aide expressed concern over assigning such a hazardous mission to a unit without combat experience, General MacArthur recalled how the 5th Cavalry had fought alongside his father's troops in the campaign against Geronimo. "They'd fight then," he said, "and they'll fight now."
Major General Charles A. Willoughby's G-2 (intelligence) section did not agree with the airmen's assessment the islands were unoccupied. Drawing on Ultra and Allied Intelligence Bureau reports from interrogating local civilians, it reported on 15 February that there were 3,000 Japanese troops in the Admiralty Islands. On 24 February, it revised the estimate to 4,000. G-2 attributed the lack of anti-aircraft fire to the Japanese logistical situation, believing it was a measure to conserve ammunition. Lieutenant General Walter Krueger, the commander of US Sixth Army later recalled no one at his headquarters believed the islands unoccupied. In the original plan, a team of Alamo Scouts was to have thoroughly reconnoitred the island before the landing. Krueger had a six-man party of Alamo Scouts inserted on the southern coast of Los Negros by PBY under cover of a bombing raid on 27 February. The scouts reported the south coast was "lousy with Japs".
Japanese defences.
The Japanese defence of the Admiralties fell under the Eighth Area Army, based at Rabaul and commanded by General Hitoshi Imamura. In September 1943, as a result of the failure to stop Allied advances in New Guinea and the Solomons, the Imperial General Headquarters (IGHQ) had decided to constrict Japan's defensive perimeter in the south and central Pacific to a new line stretching from the Banda Sea to the Caroline Islands. The IGHQ charged Imamura with holding his portion of the new line, which included the Admiralties, as long as possible to allow the Japanese navy and army time to prepare "decisive" counterattacks against Allied forces. Maintaining control of the Admiralties was crucial to the Japanese defensive plans, as possession of the islands by the Allies would place the key Japanese stronghold at Truk within range of heavy bombers. Apparently not expecting the Allies to move on the Admiralties so quickly, IGHQ gave Imamura until the middle of 1944 to complete the defensive preparations for his command. At this time the largest Japanese unit in the islands was the 51st Transport Regiment, which had arrived on Los Negros in April.
Imamura sought reinforcements for the Admiralties in late 1943 and early 1944. In October 1943 he requested an infantry division for the islands, but none was available. A subsequent proposal to transfer the 66th Regiment from the Palaus, where it was being rebuilt after suffering heavy losses, to the Admiralties was also unsuccessful as IGHQ believed that the Eighteenth Army had greater need for this unit. The Imperial Japanese Navy (IJN) also rejected Imamura's suggestion that a special naval landing force unit be dispatched to the islands. IGHQ agreed to deploy the 66th Regiment to the Admiralties in January 1944 to bolster the region's defences following the Allied landings at Arawe and Saidor in mid-December and early January respectively, but this movement was cancelled after a ship carrying reinforcements for the regiment was sunk by with heavy loss of life on the 16th of the month. Following this disaster Imamura directed the 38th Division to dispatch a battalion to the islands, and 750 men of the 2nd Battalion, 1st Independent Mixed Regiment arrived there on