On 25 June 1950 North Korea, backed by the Soviet Union and Communist China, invaded South Korea, and an American-led United Nations coalition responded by desperately holding the Pusan perimeter while General Douglas MacArthur built up forces for a successful landing at Inchon near Seoul. That assault led to a U.S./UN advance to the Yalu River, but Chinese intervention soon forced a frozen retreat back to the south. Combat Cargo Command dropped 140 tons of ammunition in two days to Marines at the Chosin Reservoir in November and airlifted out 4,600 wounded personnel. In Korea, the U.S. Air Force fought for the first time as an independent service despite old equipment, a shortage of manpower, and the harsh climate, which led Air Force Chief of Staff General Hoyt Vandenberg to call it "a shoestring Air Force." North American F-86 Sabres fought Soviet-built MiG-15s in MiG Alley over the Yalu River in the first large-scale jet-versus-jet air battles in history, and American fighter pilots prevailed because of better tactics, leadership and training. At the same time, U.S. bombers attacked North Korean and Communist Chinese forces and kept relentless pressure on North Korean supply lines. The bloody war of attrition ended in July 1953 with the Armistice, and the following year Congress passed the Air Force Academy Act of 1954. Under Lieutenant General Hubert Harmon, the first class began its four-year course in July 1955 at Denver's Lowry Air Force Base; in August 1958 the Cadet Wing was welcomed by Colorado Springs, site of today's Academy; and in 1959 the first graduating class of 207 cadets were commissioned as officers in the U.S. Air Fore.