The Grumman F4F Wildcat is an American carrier-based fighter aircraft that entered service in 1940 with the United States Navy, and the British Royal Navy where it was initially known as the Martlet. First used by the British in the North Atlantic, the Wildcat was the only effective fighter available to the United States Navy and Marine Corps in the Pacific Theater during the early part of the Second World War. The disappointing Brewster Buffalo was withdrawn in favor of the Wildcat and replaced as aircraft became available.