Answer
The abdication of Reza Shah was precipitated by the invasion of Iran by Britain and the Soviet Union in 1941. This joint operation, code-named Operation Countenance, was a response to Reza Shah's perceived pro-Axis sympathies and his refusal to expel German nationals from Iran. The invasion resulted in the occupation of Iran by Allied forces and the forced abdication of Reza Shah in favor of his son, Mohammad Reza Pahlavi.