Ernst Johannes Fritz Thälmann was a German communist politician, and leader of the Communist Party of Germany from 1925 to 1933.
A committed Marxist-Leninist, Thälmann played a major role during the political instability of the Weimar Republic, especially in its final years, when the KPD explicitly sought to overthrow the liberal democracy of the republic. Under his leadership the KPD became intimately associated with the government of the Soviet Union and the policies of Joseph Stalin, and from 1928 the party was largely controlled and funded by Stalin's government. The KPD under Thälmann's leadership regarded the Social Democratic Party as its main adversary and the party adopted the position that the social democrats were "social fascists".
Thälmann was also leader of the paramilitary Roter Frontkämpferbund. In 1932 he established Antifaschistische Aktion. He was arrested by the Gestapo in 1933 and held in solitary confinement for eleven years; for political reasons, Stalin did not seek his release when he entered into the Molotov–Ribbentrop Pact with Germany, and Thälmann's party rival Walter Ulbricht ignored requests to plead on his behalf.