Hamburger was keen to continue to Turkey as instructed, but with international tensions rising in the area the Turkish authorities reacted negatively to his Honduras passport, and he encountered difficulty obtaining a visa. Meanwhile, the situation in Iran had changed dramatically since the early summer, and the country had gained in strategic importance with regard tho the war. In August 1941, the British and the Soviets invaded and occupied Iran by a massive air, land, and naval assault. There was a concern that the shah was insufficiently anti-German and in September 1941 he had been persuaded to abdicate in favour of his son. The German invasion of the Soviet Union three months earlier had made the United States into an unlikely backer of Stalin's Soviet Union, and the Americans now raced to create a robust transport infrastructure which by 1942 had become the route by which approximately a third of all the overseas supplies to the Soviet Union were delivered. In 1943 1.6 million tons of equipment and food were delivered to the Soviet Union through Iran. The hub of the road and rail route from the sea ports to the Soviet border was Tehran. At some stage, presumably while he was still trying to arrange a visa for Turkey, Hamburger's Moscow handlers concluded that he would be more usefully deployed sending reports from Tehran, and he received instructions to stay there.