During World War II the islands were first occupied by the Japanese who established a small base in April 1942 near the village of Rossum on Manus Island.
On February 29, 1944 during World War II, the Admiralty Islands were invaded in Operation Brewer led by American General Douglas MacArthur. The Americans quickly built a big base at Seedler Harbour, including harbour, wharfes and an airbase, which became an important base for the further war operations in New Guinea as well as on the Philippines.