In early November 1936, the Nationalist Army of Africa under General Franco launched a massive assault on Madrid, aiming to capture the Republican capital and end the war. The Battle of Madrid, which continued through the winter of 1936-37, saw fierce street fighting and the heroic defense of the city by Republican forces, militiamen, and the International Brigades. The Republican government, anticipating the fall of Madrid, relocated to Valencia, but the city's defenders famously shouted '¡No pasarán!' ('They shall not pass!'), holding out against the Nationalist offensive.

The Battle of Madrid inspired the most famous slogan of the war—'¡No pasarán!'—which became a rallying cry in poetry and song. Poets like Rafael Alberti and Antonio Machado wrote powerfully about the defense of Madrid, celebrating the courage of ordinary citizens. The battle also inspired the English poet John Cornford, who wrote 'Heart of the Heartless World' on the Aragon front, capturing the apocalyptic mood of the moment.