While German armies were highly successful against the Soviet Union, the Iranian government expected Germany to win the war and establish a powerful force on its borders. It rejected British and Soviet demands to expel German residents from Iran. In response, the two Allies invaded in August 1941 and easily overwhelmed the weak Iranian army in Operation Countenance. Iran became the major conduit of Allied Lend-Lease aid to the Soviet Union. The purpose was to secure Iranian oil fields and ensure Allied supply lines, through the Persian Corridor. Iran remained officially neutral. Rezā Shāh was deposed during the subsequent occupation and replaced with his young son Mohammad Reza Pahlavi.