 The of the Imperial Japanese Army (IJA) was responsible for Imperial Japanese Army ground and air units in Southeast Asia and the South Pacific.The IJN and IJA did not formally use joint/combined staff at the operational level, and their command structures/geographical areas of operations overlapped with each other and those of the Allies.In the Pacific Ocean theater, Japanese forces fought primarily against the United States Navy, the U.S. Marine Corps and the U.S. Army.