After the Partisans managed to endure and avoid these intense Axis attacks between January and June 1943, and the extent of Chetnik collaboration became evident, Allied leaders switched their support from Draža Mihailović to Tito. King Peter II, American President Franklin Roosevelt and British Prime Minister Winston Churchill joined Soviet Premier Joseph Stalin in officially recognising Tito and the Partisans at the Tehran Conference. This resulted in Allied aid being parachuted behind Axis lines to assist the Partisans. On 17 June 1944 on the Dalmatian island of Vis, the Treaty of Vis (Viški sporazum) was signed in an attempt to merge Tito's government (the AVNOJ) with the government in exile of King Peter II. The Balkan Air Force was formed in June 1944 to control operations that were mainly aimed at aiding his forces.