With the surprise invasion of South Korea on 25 June 1950, and the tactical air forces lack of readiness due to postwar force reductions and greater emphasis on the Strategic Air Command nuclear mission in the active Air Force, most of the Air National Guard was federalized and placed on active duty.  The squadrons of the 144th Fighter Group were retained by the Air National Guard and not deployed, however numerous pilots from all three of the group's squadrons volunteered for combat duty.   The F-51Ds were exchanged for F-51H Mustangs in 1951, as the "D" model of the Mustang was needed for close air support missions in Korea.   The F-51H was a Very Long Range version of the Mustang, which was developed to escort B-29 Superfortress bombers to Japan, but not considered rugged enough to be used in Korea.  The increased range, however, was well-suited for air defense interceptor alert flights.  During its years with the P-51H, the unit earned prominence as one of the Air Force's most respected aerial gunnery competitors.  In June 1953, while still flying the Mustang, the unit qualified for the first all-jet, worldwide gunnery meet. Using borrowed F-86A Sabre jets, the 144th, which