Heavily defended by Soviet Air Forces and major anti-aircraft guns, the hydroelectric targets were subjected to attacks totaling 1,514 sorties. These were conducted jointly by fighters and fighter-bombers of the United States Air Force, US Navy, US Marine Corps, and South African Air Force, the first time in 21 months that the separate air arms had worked together on a massive scale. The attack on the facilities was followed seventeen days later by another series of large-scale joint attacks on the capital city of Pyongyang.