The route through Iran had been a major corridor for supplying goods to the Soviet Union in the American Lend-Lease program. Thus Allied troops already occupied the territory. Iran also worked well as a location on the periphery of the war, as the Soviet Union had by that time driven away the German armies in the north (after Stalingrad) and the Americans and British had defeated the German armies in North Africa. So Iran was a major location for Allied activity, and sat safely (but not too far) from the war in Europe.