The first Bf 109As served in the Spanish Civil War. By September 1939, the Bf 109 had become the main fighter of the Luftwaffe, replacing the biplane fighters, and was instrumental in gaining air superiority for the Wehrmacht during the early stages of the war. During the Battle of Britain, it was pressed into the role of escort fighter, a role for which it was not originally designed, and it was widely employed as a fighter-bomber, as well as a photo-reconnaissance platform. Despite mixed results over Britain, with the introduction of the improved Bf 109F in early 1941, the type again proved to be an effective fighter during the Invasion of Yugoslavia (where it was used by both sides), the Battle of Crete, Operation Barbarossa (the invasion of the USSR) and the Siege of Malta.