East Germany was an Eastern Bloc state under political and military control by the USSR via occupation forces and the Warsaw Pact. Although East Germany claimed to be a democracy, political power was exercised solely by leading members ("Politbüro") of the communist-controlled Socialist Unity Party of Germany, supported by the Stasi, an immense secret service. While East German propaganda was based on the benefits of the GDR's social programmes and the alleged threat of a West German invasion, many of its citizens looked to the West for freedom and prosperity. The Berlin Wall, built in 1961, prevented East German citizens from escaping to West Germany, becoming a symbol of the Cold War.