Why did the Soviet Union fight in the Spanish Civil War? International brigade soldiers with a Soviet Russian machine gun during the Spanish Civil War. Photo 12/Universal Images Group/Getty Images The USSR did a great deal to delay the fall of the Spanish Republic, but in the end could not prevent the emergence of another Hitler-friendly state in Europe. The civil war that broke out in Spain in July 1936 turned into a kind of rehearsal for WWII. It was there on the Iberian Peninsula that Nazi Germany and Fascist Italy first clashed on the battlefield with the Soviet Union.