In 1941 when the Axis invaded Yugoslavia, King Peter II formed a Government in exile in London, and in January 1942 the royalist Draža Mihailović became the Minister of War with British backing. But by June or July 1943, British Prime Minister Winston Churchill had decided to withdraw support from Mihailović and the Chetniks he led, and support the Partisans headed by Josip Broz Tito, even though this would result in "complete communist control of Serbia". The main reason for the change was not the reports by Fitzroy Maclean or William Deakin, or as later alleged the influence of James Klugmann in Special Operations Executive (SOE) headquarters in Cairo or even Randolph Churchill, but the evidence of Ultra decrypts from the Government Code and Cipher School in Bletchley Park that Tito's Partisans were a "much more effective and reliable ally in the war against Germany". Nor was it due to claims that the Chetniks were collaborating with the enemy, though there was some evidence from decrypts of collaboration with Italian and sometimes German forces.