 Nagy was a committed communist from soon after the Russian Revolution, and through the 1920s he engaged in underground party activity in Hungary.Living in the Soviet Union from 1930, he served the Soviet NKVD secret police as an informer from 1933 to 1941, denouncing over 200 colleagues, who were then purged and arrested and 15 of whom were executed.Nagy returned to Hungary shortly before the end of World War II, and served in various offices as the Hungarian Working People's Party (MDP) took control of Hungary in the late 1940s and the country entered the Soviet sphere of influence.