While the idea of vertical flight - the helicopter - was envisioned as early as the 15th century by Leonardo da Vinci, the first actual helicopter flight was in 1924 and the first practical helicopter was the FW 61 in Germany in 1936 with a small number used during World War II. The U.S. used the Sikorsky VS 300 in World War II. Marine General Roy Geiger after watching an A bomb test in 1946 determined the need for helicopters for troop landings in the future in a battlefield with nuclear arms. The use of helicopters in the Korean War were instrumental in providing supply and troop movement, evacuation of wounded and rescue of downed pilots. Vietnam in the 1960's and 1970's became the first true helicopter war. By the end of the Vietnam War the Army and Marine helicopter fleet size made it the world's third largest air force after the U.S. and USSR air forces. The Huey of Vietnam was eventually replaced with the Blackhawk in 1978 with its first combat deployment in Grenada in 1983. Several Blackhawk variants are currently in use by the Army, Navy, Air Force and Coast Guard. The stealth version of the Blackhawk has been instrumental in the war on terror including use in the May 2011 operation that killed bin Laden.