The military conspiracy against Spain's Second Republic was supposed to seize Madrid in a swift coordinated action. Instead, it unraveled before it began. Colonel Valentin Galarza Morante, the plot's coordinator in the capital, was detained by government agents. His replacement, the elderly General Villegas, lost his nerve at the last moment and stepped aside. General Joaquin Fanjul, brought in at short notice, found himself leading an uprising with no plan, no coordination between the falangists, monarchists, and army officers who were supposed to act together, and a city bristling with pro-government security forces. Twenty-five companies of Assault Guards and fourteen of Civil Guards were either stationed in Madrid or had been brought in by Republican authorities who sensed trouble. Against this, Fanjul had perhaps 2,000 officers, cadets, and soldiers plus 500 civilian volunteers holed up in a barracks built in 1860.

On July 18, news of the uprising in Spanish Morocco reached Madrid and the unions demanded arms. The government initially refused to arm civilians, but Lieutenant Colonel Rodrigo Gil Ruiz distributed 5,000 rifles to workers anyway. The next morning, Prime Minister Jose Giral authorized the distribution of 65,000 more -- useless without the bolts Colonel Serra was withholding. Fanjul attempted to march his troops out of the barracks into the city center but retreated when he saw eight thousand organized workers, some armed, surrounding the complex. Assault Guards took up positions on neighboring rooftops. The coup had already failed at every other garrison in the city: at Carabanchel, General Garcia Herran was killed by his own troops. At Retiro, the garrison surrendered without opposition. At Getafe Air Base, rebels were defeated after an officer's death, freeing aircraft to fly over the Montana Barracks.

On the morning of July 20, artillery joined the siege -- two 75mm guns and one 155mm gun, commanded by a retired captain named Orad. A Breguet XIX biplane from Cuatro Vientos Air Base dropped bombs on the barracks. At half past ten, one bomb wounded both Fanjul and Serra. Soldiers inside waved a white sheet from the windows. The crowd, interpreting this as surrender, surged forward against the orders of Lieutenant Moreno of the Assault Guards. From other windows, defenders opened fire with machine guns, cutting down the advancing civilians. This happened twice. The false surrenders -- whether deliberate treachery or confused signals from a garrison tearing itself apart -- transformed the crowd's determination into rage. Around noon, the attackers broke through the main gate. What followed was a massacre. Some surrendering defenders were killed in the main courtyard, several thrown from an upper gallery. A group of rebel officers gathered in a mess room and shot themselves rather than face the crowd.

Of the 145 rebel officers who had gathered at the Montana Barracks, 98 died -- in combat, killed after surrendering, by their own hand, or by later execution. Colonel Serra was among those killed immediately after the fall. The wounded General Fanjul was taken to Madrid's Model Prison, tried for military rebellion, and executed. Total losses among the defenders are estimated between 200 and 1,000 dead. Some falangist and monarchist volunteers in civilian clothes slipped away in the chaos. The rifle bolts and ammunition -- the immediate cause of the entire confrontation -- were seized by Assault Guards and delivered to the Ministry of War. The damaged barracks stood in ruins for years before being demolished. Today the site is a public garden on Calle de Ferraz, its trees growing where courtyard flagstones once ran with blood. No monument at the site explains what happened there in July 1936, though the events shaped everything that followed in the Spanish Civil War.