In August 1940, Leon Trotsky, one of the architects of Bolshevism and the Russian Revolution, was assassinated in Mexico City, where he lived in exile. It was the second attempt on his life in less than three months. The murderer, Ramón Mercader del Río, was a Spanish Communist whose orders came directly from JosephStalin.

Stalin had banished Trotsky from the Soviet Union in 1928, but Trotsky continued to write harsh criticisms of the revolution’s direction under Stalin’s increasingly tyranni...