During the Spanish Civil War in August 1936, Nationalist soldiers captured Republican soldiers at the Somosierra Pass in the Guadarrama Mountains north of the capital Madrid. Nationalist forces guarded the surrendered Republicans, rifles tucked under their arms, on a battlefield littered with corpses.

　　During the Spanish Civil War in August 1936, Nationalist soldiers captured Republican soldiers at the Somosierra Pass in the Guadarrama Mountains north of the capital Madrid. Nationalist forces guarded the surrendered Republican soldiers, rifles tucked under their arms, on a battlefield littered with corpses. During the Battle of the Somosierra Pass, Carlist and Falangist units clashed fiercely with Republican forces on the pass's high ground, resulting in the Nationalists taking Republican soldiers prisoner. Republican militiamen entrenched in trenches attempted to prevent Nationalist forces from crossing the mountains. The Republican government deployed regular army soldiers, but thousands of irregular fighters, including Communist militiamen, also joined the battle. The Battle of Somosierra Pass in the Guadarrama Mountains was one of the early engagements of the Spanish Civil War.

　　The Spanish Civil War began on July 18, 1936, when right-wing military officers revolted in Spanish Morocco, and the conflict spread to mainland Spain. General Francisco Franco called on all military officers to join the rebellion via a broadcast from the Canary Islands, urging the overthrow of Spain's left-wing Republican government. Three days later, rebel forces occupied Morocco, most of northern Spain, and several major cities in the south. In other cities, including the capital Madrid, Republican forces succeeded in suppressing the rebellion. The Republicans and the “Nationalists” (the term for the rebel forces) each secured areas of control. 

　　During the Spanish Civil War, conservative forces within Spain supported a military rebellion against the republican government. After the initial military coup failed to establish nationwide control, a bloody civil war erupted, with both sides engaging in fierce combat. The fascist governments of Italy and Germany supported the rebel forces, known as the National Army. The Republicans received support from the Soviet Union and the International Brigades, composed of volunteers from Europe and the Americas. The Republican government surrendered Madrid on March 28, 1939, ending the Spanish Civil War. This conflict became the bloodiest in Spanish history, with casualties reaching up to one million. Republican forces suffered 110,000 combat deaths, while 50,000 to 200,000 civilians were killed in Nationalist-held areas. Nationalist forces suffered 90,000 combat deaths, and 38,000 to 110,965 civilians were killed in Republican-held areas.