The invasion of the Admiralty Islands (29 February-25 March 1944) was a major step in the isolation of the powerful Japanese base at Rabaul, and saw forces from the US Cavalry capture the main islands in a series of battles that lasted for one month. The conquest of the Admiralty Islands helped complete the isolation of Rabaul by giving the Allies control of the western approaches to New Britain, as well as a useful site for airfields and control of the massive Seeadler Harbour.
The Admiralty Islands sit at the north-west corner of the Bismarck Sea, west of the Bismarck Archipelago (New Britain and New Ireland) and north of New Guinea. Allied control of the islands would thus help further isolate Rabaul, at the northern tip of New Britain, as well as providing bases for the Allied advance along the north coast of New Guinea. A further bonus would be control of the massive Seeadler Harbour, 20 miles long and 6 miles wide, a harbour capable of sheltering an entire fleet.
The Japanese had recognised the importance of the Admiralty Islands. They were defended by 4,300 men, commanded by Colonel Yoshio Ezaki. They had also built an airfield on the largest island, Manus, and on nearby Los Negros. When the Americans attacked they would be outnumbered, but Ezaki expected the attack to come from Seeadler Harbour and so his defences were all facing in, towards the harbour.
The Admiralty Islands had been an American target for some time, and the invasion was originally timetabled for April 1944. Early in 1944 aircrews from the Fifth Air Force reported that there was no Japanese activity on the islands. In addition MacArthur was impatient to speed up his advance towards the Philippines. He decided to push the invasion forwards to late February 1944, but also realised that the aerial reconnaissance might be inaccurate. He decided to make the original landings a 'reconnaissance in force' and to accompany the task force in person so he could decide if the islands could be held or not.
The decision to invade was made on 24 February, with D-Day set for 29 February. Los Negros would be the first target, and the landing would be carried out by 1,000 men from the 1st Cavalry Division (Dismounted). A reserve of 1,500 men was also available. The landings would take place at Hyane Bay, on the east, ocean-facing, coast of Los Negros.
The Admiralty Islands contains eighteen main islands. The largest island, by far, is Manus. The island is about 100km/ 60 miles from west to east and 30km/ 18 miles from north to south. At the eastern tip of Manus is Los Negros, the third biggest of the islands. This island takes the shape of a horseshoe, curving around to the north then to the west. A series of small islands then run west from the north-western tip of Los Negros, running parallel to the north-eastern coast of Manus. The area inside this line of islands is Seeadler Harbour.
On 27 February MacArthur boarded the USS Phoenix, the flagship of the invasion forces. On the same day a force of six scouts landed on Los Negros. They landed safely, but found that the island was 'lousy' with Japanese troops. The news reached MacArthur at sea on 28 February, but he decided to press away with the invasion anyway. On the morning of 29 February the 1st Cavalry landed on Los Negros. The Japanese were caught entirely out of position, and the Americans quickly established a beachhead. That afternoon MacArthur paid a two hour visit to the island, and decided that his men should stay. He then left, taking most of the naval support force with him.
That night the Japanese launched an attack on the beachhead, but the Americans held their ground. In some areas the Japanese did manage to get inside the perimeter, and on the following morning the area had to be re-secured. Sixty six Japanese troops were killed within the main perimeter.
Two days later reinforcements arrived, just in time to help repulse a second major Japanese attack on the night of 3-4 March. This was a more serious attack, and the Americans lost 61 dead while fighting it off. Japanese losses were much heavier, with 750 dead around the perimeter. This was the last major Japanese attack on Los Negros. The last major fighting came on 21-25 March, when the Japanese were forced out of their last organised defences, on a ridge near Papitalai
Manus Island was the next target. The Manus task force entered Seeadler Bay on 9 March and began by occupying the islands that ran west from the northern end of Los Negros, parallel to Manus. The Japanese put up unexpectedly fierce resistance on Hauwei, but the island was secured on 12 March.
On 15 March the Americans landed at Lugos Mission, west of their main target at Lorengau. Lorengau airfield fell on 17 March, and the town fell soon afterwards. The hardest fighting came when the Americans began to advance south from Lorengau towards Rossum. This battle lasted from 19-25 March, and ended organised Japanese resistance in the Admiralty Islands. The campaign officially came to an end on 18 May 1944, but by then the only Japanese left were isolated bands of stragglers.
The two battles cost the Americans 326 dead and 1,200 wounded. The Japanese lost 3,280 dead.
On New Guinea the fall of the Admiralty Islands convinced General Adachi that he could no longer hold onto his base at Madang. He ordered his men to march west to Hansa Bay and Wewak, where he expected the next American blow to fall. Allied trooped made an unopposed entry into Madang on 24 April, two days after they had leapfrogged Adachi yet again, landing at Aitape and Hollandia, half way along the north coast of New Guinea.
The Admiralty Islands also became an important American base. Seeadler Harbour was a very useful naval base. In addition a PT base was built at the northern tip of Los Negros, with a 8,000ft airstrip nearby and a 7,000ft airstrip was completed at Momote, near the original landing point at Hyane Harbour.