General Franco took command of a conservative military rebellion in 1936 against the republican government in Madrid, which was supported by socialist and anarchist groups. Both fascist Italy under Benito Mussolini and Nazi Germany sent troops to support Franco, while the government got support from the Soviet Union as well as from individual volunteers from several countries. The war was determined when Franco conquerred Catalonia in January/February 1939. Madrid was sieged without resistance on March 28, 1939.
Nazi Germany also used the war as a testing ground for new strategy, Blitzkrieg, to take advantage of faster tanks and aircraft that were just becoming available at the time.
Many foreigners were involved in the war on both sides. Ernest Hemingway fought on the side of the government.