Russian revolutionary Sergei Kirov murdered

On December 1, 1934, Leningrad mayor Sergey Kirov was assassinated and Joseph Stalin used it as an excuse to begin his Great Purge of 1934-38. From the article:

"Russian revolutionary Sergei Kirov murdered
Sergei Kirov, a leader of the Russian Revolution and a high-ranking member of the Politburo, is shot to death at his Leningrad office by Communist Party member Leonid Nikolayev, likely at the instigation of Soviet leader Joseph Stalin.

Whatever Stalin’s precise role in the assassination of his political rival Kirov, he used the murder as a pretext for eliminating many of his opponents in the Communist Party, the government, the armed forces, and the intelligentsia. Kirov’s assassination served as the basis for seven separate trials and the arrest and execution of hundreds of notable figures in Soviet political, military, and cultural life. Each trial contradicted the others in fundamental details, and different individuals were found guilty of organizing the murder of Kirov by different means and for varying political motives.

The Kirov assassination trials marked the beginning of Stalin’s massive four-year purge of Soviet society, in which millions of people were imprisoned, exiled or killed."