Post-World War II development of conventionally armed attack aircraft in the United States had stagnated. Design efforts for tactical aircraft focused on the delivery of nuclear weapons using high-speed designs like the F-101 Voodoo and F-105 Thunderchief. Designs concentrating on conventional weapons had been largely ignored, leaving their entry into the Vietnam War led by the Korean War-era Douglas A-1 Skyraider. While a capable aircraft for its era, with a relatively large payload and long loiter time, the propeller-driven design was also relatively slow and vulnerable to ground fire. The U.S. Air Force and Marine Corps lost 266 A-1s in action in Vietnam, largely from small arms fire. The A-1 Skyraider also had poor firepower.