In 1942, Fred J Christensen joined the United States Army Air Forces as an aviation cadet.
Christensen was credited with his first German aircraft shot down on November 26, 1943, a Messerschmitt Bf 110 near Papenburg, Germany. He was recognized as an ace on February 11, 1944, with the crediting of his fifth kill, a Bf 109. He continued to regularly shoot down German aircraft, scoring two kills on the first day of Big Week, another on a long-range bomber escort to Berlin on March 6, and four more in consecutive missions of March 15 and 16.
Promoted to first lieutenant in February and captain in April, Christensen shot down his 16th aircraft on July 5, and also that day incurred the first battle damage of his tour. On July 7, leading the 62nd FS, Christensen was returning from a bomber escort mission and overflew Gardelegen Airfield, assessing it as a possible strafing target. Christensen noted numerous parked aircraft dispersed on the field, but then observed a flight of Junkers Ju 52 transports in trail at very low altitude, approaching to land.
Christensen dove on the landing pattern from 10,000 feet (3,000 m) and shot down the next-to-last transport with hits on the left side. He quickly overtook and fired at a second target, which burned and crashed. A third transport attempted to evade to the left, but he downed it with a deflection burst, setting its fuel tanks on fire. The German pilot attempted to land, but crashed short of the field.
As Christensen maneuvered for a fourth interception, his engine quit from fuel starvation. He scored several hits, and in its attempt to evade, the transport flew into the ground. Christensen attacked a fifth airplane, observing strikes on its fuselage and wing root, and shot it down before it could land. He then destroyed a sixth Ju 52 to become an "ace in a day" in an engagement that had lasted less than two minutes. In all, 10 of the 12 transports were shot down by the 62nd FS.
Christensen flew 107 combat sorties with the 56th Fighter Group and used five different aircraft to record his victories, including two assigned as his personal aircraft.