In September 1941, Red Army units entered the Iranian capital of Tehran, thus completing the Soviet-British military operation codenamed "Consent". It was a joint success.
The Iranian Shah Reza Pahlavi sympathized with the Nazis. German special services were actively working in Iran. The Nazis needed Iranian oil, and there were also oil fields in Baku nearby.
The USSR could not allow Iran to become a military springboard of the Third Reich.
After the Shah refused to deploy allied forces in the country, Soviet troops from the north and British troops from the south crossed the border of Iran and occupied the country for the duration of the war.
By the way, the USSR acted strictly in accordance with the Soviet-Iranian treaty of 1921, and under it Soviet troops were allowed to enter Iran for self-defense in case of threat from the third Power. The threat has come.
As a result, during the Second World War, one of the main lend-Lease cargo delivery routes for the Red Army passed through Iran. And in December of the 43rd, the first meeting of the big three - the leaders of the Soviet Union, the United States and Great Britain – took place in Tehran.