What became known as the Spanish Civil War began on 17 July 1936 when a group of right-wing generals staged a military coup, aimed at overthrowing Spain's democratically elected government. The coup was supported by military units in many cities around Spain. However, some of the country's biggest cities, such as Madrid, Barcelona and Bilbao, remained under government control.

Within days of the coup, Hitler and Mussolini intervened on the side of the insurgent generals. The Soviet Union came to the aid of the Loyalists, also known as the Republican forces. Britain, France and the US decided to remain neutral, even imposing – and enforcing – an embargo on the sale of arms to the Republic.

The battle for control of Spain turned into a horrific conflagration that lasted almost three years. The new medium of photojournalism brought horrifying images of the conflict into homes around the world. In an unprecedented display of international solidarity, some 35,000 volunteers from 50 nations joined the International Brigades and risked life and limb to defend the Republic.

From their strongholds in the south and west, the Nationalists – supported by Italian and Moroccan troops – advanced northwards, capturing cities from Republicans weakened by the embargo. Barcelona and the rest of Catalonia fell in early 1939, and Franco's troops marched into Madrid in April.

Historians estimate as many as 500,000 combatants and civilians were killed on the Republican and Nationalist sides in the war. Atrocities were committed on both sides. The victors under General Franco executed thousands more after the war. It is estimated that over 100,000 men and women were executed and buried in unmarked graves during the conflict.