 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.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.