 By late 1937, all remnants of collective leadership were gone from the Politburo, which was controlled entirely by Stalin.There were mass expulsions from the party, with Stalin commanding foreign communist parties to also purge anti-Stalinist elements.During the 1930s and 1940s, NKVD groups assassinated defectors and opponents abroad; in August 1940, Trotsky was assassinated in Mexico, eliminating the last of Stalin's opponents among the former Party leadership.