Erich Mielke, who used the cover name "Fritz Leissner." In addition to Zaisser and Mielke, the S.I.M. was filled with countless other agents of the GRU and NKVD, whose Spanish rezident was General Aleksandr Mikhailovich Orlov. According to author Donald Rayfield, "Stalin, Yezhov, and Beria distrusted Soviet participants in the Spanish war. Military advisors like Vladimir Antonov-Ovseenko, journalists like Koltsov were open to infection by the heresies, especially Trotsky's, prevalent among the Republic's supporters. NKVD agents sent to Spain were therefore keener on abducting and murdering anti-Stalinists among Republican leaders and International Brigade commanders than on fighting Franco. The defeat of the Republic, in Stalin's eyes, was caused not by the NKVD's diversionary efforts, but by the treachery of the heretics." Zaisser quickly achieved the rank of brigadier general (initially commanding XIII International Brigade), and in 1937, he became leader of all the pro-Republican International Brigades operating in Spain. Following the end of the Spanish Civil War in 1939, Zaisser returned to Moscow to resume working for the Comintern, but was thrown into jail, apparently because of the failure of the Soviet intervention in Spain. During and after World War II, Zaisser taught Stalinist re-education courses to German prisoners of war. Government In 1947, Zaisser returned to Germany and joined the Socialist Unity Party (SED). Zaisser's career took off rapidly soon afterwards, and by 1948 he was Minister of the Interior and Deputy Minister-President of Saxony-Anhalt. From 1949 to 1954, Zaisser served as a representative in the Volkskammer and in 1950 worked on military and tactical issues at the Marx-Engels-Lenin-Stalin Institute, a facility to which very few non-Soviets had access. In 1950, Zaisser gained membership in East Germany's Politburo and the Central Committee of the SED, thus becoming one of the most powerful men in the country. In the same year, Zaisser was awarded the Karl Marx Medal and appointed Director of the Ministry of State Security. Using his vast knowledge of intelligence work, Zaisser built the Stasi into a powerful organization. After the death of Soviet leader Joseph Stalin on 5 March 1953, Moscow favored replacing East Germany's Stalinist party leader Walter Ulbricht and considered Zaisser a potential candidate. However, the workers' uprising, which was suppressed by the Red Army on 17 June, led to a backlash. Alarmed by the uprising, Lavrenty Beria, the First Deputy Premier of the Soviet Union and head of the Ministry of Internal Affairs,