On 6 June 1944, Allied forces stormed the beaches at Normandy. The invasion followed several years of argument and planning by Allied leaders, who remained committed to a return to the European continent after the Germans had forced the Allies to evacuate at Dunkirk in May 1940. Before the spring of 1944, however, Prime Minister Winston Churchill and other British leaders remained unconvinced that the invasion was feasible. At the Teheran Conference in November 1943, Franklin D. Roosevelt and Winston Churchill promised Josef Stalin that Allied troops would launch Operation Overlord, the invasion of Normandy, in the spring. Because of their continuing concerns about Overlord, the British convinced the Americans to implement a cover plan to help ensure the invasion's success. The London Controlling Section (LCS) devised an elaborate two-part plan called Operation Fortitude that SHAEF (Supreme Headquarters, Allied Expeditionary Force) helped to fine tune and that both British and American forces implemented. Historians analysing the Normandy invasion frequently devote some discussion to Operation Fortitude. Although they admit that Fortitude North did not accomplish all that the Allied deception planners had hoped, many historians heap praise on Fortitude South, using phrases such as, "unquestionably the greatest deception in military history." Many of these historians assume that the deception plan played a crucial role in the June 1944 assault. A re-examination of the sources suggests, however, that other factors contributed as much, if not more, to the Allied victory in Normandy and that Allied forces could have succeeded without the elaborate deception created by the LCS. Moreover, the persistent tendency to exaggerate the operational effect of Fortitude on the German military performance at Normandy continues to draw attention away from other, technical-military reasons for the German failures there.