After Germany broke its pact with the Soviets and invaded the USSR in June 1941, the United Kingdom and the Soviet Union jointly occupied neutral Iran as a preventive measure, starting on August 25, 1941, and justified their invasion by the need to use Iran as a gateway for delivery of Lend-Lease supplies to the Soviet Union from British India. Iran had laid torn between the concurrent zones of influence of both Britain and Russia for a long time, but had managed until then to remain independent, despite several foreign interventions, by taking advantage of the rivalry between the two rival powers. Now that they stood together against Germany, nothing longer opposed a formal joint occupation of the country. As a result, Rezā Shāh was forced to abdicate on September 16, 1941 and exiled to Mauritius; his son, Mohammad Reza Pahlavi, the crown prince, became the new monarch. Their joint 'Tripartite Treaty' of January 1942 declared that their military presence was not an occupation, proclaimed Iran to be their ally, and pledged to withdraw their troops within six months of the end of the war.