During the 1930s and 1940s , Joseph Stalin ' s NKVD carried out numerous assassinations outside of the Soviet Union , such as the killings of Organization of Ukrainian Nationalists leader Yevhen Konovalets , Ignace Poretsky , Fourth International secretary Rudolf Klement , Leon Trotsky , and the Workers ' Party of Marxist Unification ( POUM ) leadership in Catalonia Michael Ellman .