After being designated a Naval Aviator and commissioned as a second lieutenant, Foss served as an instructor at Pensacola, Florida and later attended the Navy School of Photography, at which time he was assigned to Marine Photographic Squadron 1 (VMO-1) which was stationed at Naval Air Station North Island in San Diego, California. Eager for combat, he qualified in Grumman F4F Wildcats while still assigned to VMO-1 and was eventually transferred to Marine Fighting Squadron 115 VMF-115 as the executive officer. In October 1942, VMF-115 was deployed to the South Pacific and became part of the Cactus Air Force in the Battle of Guadalcanal with Foss serving as the executive officer. On combat missions, he led a flight of eight Wildcats that became known as Foss's Flying Circus. He shot down a Japanese Zero in his first combat on October 13, but his own plane was hit and, with a dead engine and three more Zeros on his tail, he landed at full speed, no flaps and minimal control on the American runway at Guadalcanal, barely missing a grove of palm trees. By the time Foss left Guadalcanal in January 1943, his Flying Circus had shot down 72 Japanese aircraft, including 26 credited to him. As America's "ace of aces", he received the Medal of Honor during a White House ceremony in 1943, and appeared on the cover of Life magazine.