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.

The Admiralty Islands are an archipelago group of 18 islands in the Bismarck Archipelago, to the north of New Guinea in the South Pacific Ocean. These are also sometimes called the Manus Islands, after the largest island.

These rainforest-covered islands form part of Manus Province, the smallest and least-populous province of Papua New Guinea, in its Islands Region. The total area is 2,100 km2 (810 sq mi). Many of the Admiralty Islands are atolls and uninhabited.