It was activated under Fourth Air Force at March Field, California in June 1941, when it replaced a provisional organization.

It was responsible for training fighter units and for the air defense of the southern portion of the Pacific Coast.

[1][note 3] The attack on Pearl Harbor put all units in the Western Theater of Operations on heightened alert.

[4] The command was charged with control of "active agents" for air defense in its area of responsibility, which included interceptor aircraft, antiaircraft artillery and barrage balloons.

[6] In June it moved its headquarters north to Oakland Airport, California, which was more centrally located for its increased area of responsibility.

[1] Along the Pacific coast, Western Defense Command established a "vital air defense zone", extending from the coast approximately 150 miles (240 km) inland and 200 miles (320 km) to sea, with long range bombers from 4th Bomber Command flying patrols over the ocean.

[9] As the possibility of an attack on the Pacific coast grew more remote, the air defense wings became increasingly concerned with the training mission.

[14] Shortly thereafter, it was decided that one of the command's groups would be manned at 50% overstrength in order to train and supply replacement pilots for overseas units.

[18] In connection with this general reorganization, Fourth Air Force disbanded IV Fighter Command and transferred its responsibilities to wings that were headquartered by AAF base units.