The war began after a pronunciamiento (declaration of opposition) by a group of generals of the Spanish Republican Armed Forces, originally under the leadership of José Sanjurjo, against the elected, leftist government of the Second Spanish Republic, at the time under the leadership of President Manuel Azaña. The Nationalist group was supported by a number of conservative groups, including the Spanish Confederation of Autonomous Right-wing Groups (Confederación Española de Derechas Autónomas, or CEDA), monarchists such as the religious conservative (Catholic) Carlists, and the Falange Española Tradicionalista y de las Juntas de Ofensiva Nacional Sindicalista, a fascist group. Sanjurjo was killed in an aircraft accident while attempting to return from exile in Portugal, whereupon Franco emerged as the leader of the Nationalists.
Members of one of the Republican International Brigades at the Battle of Belchite ride on a T-26 tank
The coup was supported by military units in the Spanish protectorate in Morocco, Pamplona, Burgos, Zaragoza, Valladolid, Cádiz, Córdoba, and Seville. However, rebelling units in important cities—such as Madrid, Barcelona, Valencia, Bilbao, and Málaga—were unable to capture their objectives, and those cities remained under the control of the government. Spain was thus left militarily and politically divided. The Nationalists and the Republican government fought for control of the country. The Nationalist forces received munitions and soldiers from Nazi Germany and Fascist Italy, while the communist Soviet Union and socialist Mexico offered support to the "Loyalist" or "Republican" side. Other countries, such as the United Kingdom and France, operated an official policy of non-intervention, but "substantial unofficial aid flowed from France to the Loyalists."
The Nationalists advanced from their strongholds in the south and west, capturing most of Spain's northern coastline in 1937. They also besieged Madrid and the area to its south and west for much of the war. Capturing large parts of Catalonia in 1938 and 1939, the war ended with the victory of the Nationalists and the exile of thousands of leftist Spaniards, many of whom fled to refugee camps in southern France. Those associated with the losing Republicans were persecuted by the victorious Nationalists. With the establishment of a dictatorship led by General Francisco Franco in the aftermath of the war, all right-wing parties fused into the structure of the Franco regime.
The war became notable for the passion and political division it inspired and for the many atrocities. Organized purges occurred in territory captured by Franco's forces to consolidate the future regime. A significant number of killings took place in areas controlled by the Republicans. The extent to which Republican authorities took part in Republican territory killings varied.
(source)