On the 12th of July 1936, a single act of political murder transformed a simmering political crisis into a full-scale civil war that would consume Spain for three years. Lieutenant José Castillo, a Socialist party member and Assault Guard officer, was assassinated by Falangist gunmen in Madrid. This was not an isolated incident but the culmination of months of escalating violence, yet it served as the immediate catalyst for the military uprising that had been brewing since the Popular Front's election victory earlier that year. The murder of Castillo triggered a chain reaction: his friend, Assault Guard Captain Fernando Condés, led a squad to arrest prominent conservative politician José Calvo Sotelo, who was not at home. Instead, they found him and summarily executed him. The killing of a parliamentary leader by state police was unprecedented in modern Spanish history and shattered any remaining faith in the government's ability to maintain order. Within days, the military conspirators, who had been planning a coup d'état, found their justification solidified. General Emilio Mola, the chief planner of the rebellion, received confirmation from General Francisco Franco, who had been transferred to the Canary Islands, that the time had come to act. The assassination of Calvo Sotelo transformed a limping conspiracy into a revolt that could trigger a civil war, as the belief that the state had ceased to be neutral and effective encouraged important sectors of the right to join the rebellion. The timing of the uprising was fixed for the 17th of July, but the discovery of the plot in Spanish Morocco forced the conspirators to enact it immediately, beginning the bloodiest conflict in Spanish history.
The Army Divided Against Itself
The Spanish military was not a monolithic force but a deeply fractured institution divided by geography, generation, and ideology. Of the approximately 15,301 officers serving in the Spanish army, just over half rebelled against the Popular Front government, while the rest remained loyal. This division was not random but followed a distinct pattern based on where officers had served their early careers. Officers who had fought in North Africa between 1909 and 1923, known as africanistas, tended to support the coup, while those who had remained in mainland Spain, called peninsulares, generally stayed loyal. The africanistas had benefited from a suspension of traditional promotion by seniority in favor of promotion by merit through battlefield heroism, allowing younger officers to leapfrog through the ranks. This created deep resentment among the peninsulares, who viewed the africanistas as swaggering and arrogant. The coup divided regular forces fairly evenly, with some 52 percent of the 66,000 military personnel under arms in July 1936 joining the Republican zone and 48 percent joining the Nationalist one. However, the rebels secured control of the Army of Africa, made up of 35,000 men, which became the core of their fighting force. The Spanish Legion, composed of these African troops, committed atrocities and carried out summary executions of leftists, while the Moroccan Fuerzas Regulares Indígenas joined the rebellion and played a significant role in the civil war. The outcome of the initial coup was that the rebels failed to take any major cities with the critical exception of Seville, which provided a landing point for Franco's African troops. The government retained control of Málaga, Jaén, and Almería, while the rebels took Cádiz with help from the first troops from Africa. The air bridge between Tetouan and Seville with German planes allowed the Army of Africa to advance towards Extremadura and Madrid. The result of the coup was a nationalist area of control containing 11 million of Spain's population of 25 million, while the Republicans controlled under half of the rifles and about a third of both machine guns and artillery pieces. The Spanish Republican Army had just 18 tanks of a sufficiently modern design, and the Nationalists took control of 10. Naval capacity was uneven, with the Republicans retaining a numerical advantage, but with the Navy's top commanders and two of the most modern ships, heavy cruisers Canarias and Baleares, in Nationalist control.