During the Second World War in Yugoslavia, the United States initially supported the royal government of Yugoslavia. When the Nazis invaded Yugoslavia in the spring of 1941, the United States provided large amounts of support to the Chetniks in the first years of the war. This support took place in the form of extensive clandestine relations between the Office of Strategic Services and Chetniks with William Donovan's administration. Such cooperation was highlighted by complex operations such as Operation Halyard, in which several hundred American pilots were rescued by Chetniks.