In 1943 the U.S. Armed Forces temporarily took over Oakland Airport and opened Naval Air Station Oakland. It was transformed into an airlift base for military flights to the Pacific islands, ordering all scheduled service to move to San Francisco International Airport. After the war, airlines slowly returned to Oakland; Western Airlines began flights in 1946, and was followed by American Airlines, TWA, United, Transocean Air Lines and Pacific Southwest Airlines (PSA). Oakland became a larger center for non-scheduled and supplemental air carriers: The four largest such companies were based and headquartered in Oakland: Transocean Air Lines, World Airways, Trans International Airlines and Saturn Airways. Oakland became one of the largest overhaul and maintenance operations in the country when Aircraft Engine and Maintenance Company (AEMCO) processed thousands of aircraft.