Short answers, pulled from the story.
Ja'far Pishevari led the Azerbaijan People's Government which was proclaimed in Tabriz in 1945. Lavrenti Beria was nominally in charge of the operation but delegated authority to Mir Jafar Baghirov.
The Azerbaijan People's Government existed from November 1945 until December 1946. The Iranian army reentered Tabriz by mid-December 1946 ending the state after one year of existence.
Soviet forces captured Tabriz and northwestern Iran for military reasons following the dethronement of Reza Shah in September 1941. The Soviets set up the government under Ja'far Pishevari to expand influence while preventing the Iranian army from intervening during expansion.
Approximately 500 supporters were killed during the lawless interregnum before troops arrived in Tabriz. A British source cited by the US Embassy in Tehran gave the number of killed Democrats as 421.
The U.S. supported the Iranian complaint against Soviet actions lodged with the Security Council to protect Iran's natural resources. Resolution 3 and Resolution 5 were passed during mid December 1946 leading to U.S. backing of the shah's government sending the Iranian army to re-occupy Mahabad and Azerbaijan.