The Nationalists (nacionales) received the support of Fascist Italy and Nazi Germany, as well as neighbouring Portugal. The Communist Soviet Union intervened on the Republican side, although it encouraged factional conflict to the benefit of the Soviet foreign policy, and its actions may have been detrimental to the Republican war effort as a whole. The United States government offered no official support for either side, although over two thousand Americans volunteered on the Republican side. Meanwhile, American corporations such as Texaco, General Motors, Ford Motors, and The Firestone Tire and Rubber Company greatly assisted the Nationalist army, furnishing a regular supply of trucks, tires, machine tools, and fuel.

The new tank warfare tactics and the terror bombing of cities from the air were features of the Spanish Civil War which played a significant part in the later general European war.

The Spanish Civil War has been dubbed "the first media war", with several writers and journalists covering it wanting their work "to support the cause". Foreign correspondents and writers covering it included Ernest Hemingway, Martha Gellhorn, Cesar Vallejo, George Orwell, Halfdan Rasmussen and Robert Capa. Like most international observers, they tended to support the Republicans, with some such as Orwell participating directly in the fighting.

Like most civil wars, it became notable for the passion and political division it inspired, and for atrocities committed on both sides of the conflict. The Spanish Civil War often pitted family members, neighbors, and friends against each other. Apart from the combatants, many civilians were killed for their political or religious views by both sides, and after the war ended in 1939, the losing Republicans were persecuted by the victorious Nationalists.

An estimated total of more than 300,000 people lost their lives as a consequence of the war. Out of them probably more than 120,000 were civilians executed by either side.

There were several reasons for the war, many of them long-term tensions that had escalated over the years.

The 19th century was turbulent for Spain. The country had undergone several civil wars and revolts, carried out by both reformists and the conservatives, who tried to displace each other from power. A liberal tradition that first ascended to power with the Spanish Constitution of 1812 sought to abolish the monarchy of the old regime and to establish a liberal state. The most traditionalist sectors of the political sphere systematically tried to avert these reforms and to sustain the monarchy. The Carlists—supporters of Infante Carlos and his descendants—rallied to the cry of "God, Country and King" and fought for the cause of Spanish tradition (monarchy and Catholicism) against the liberalism – and later, the republicanism – of the Spanish governments of the day. The Carlists, at times (including the Carlist Wars), allied with nationalists (not to be confused with the nationalists of the Civil War) attempting to restore the historic liberties (and broad regional autonomy) granted by the fueros (regional charters) of the Basque Country and Catalonia. Further, from the mid-19th century onwards, liberalism was outflanked on its left by socialism of various types and especially by anarchism, which was far stronger in Spain than anywhere else in Europe.

Spain experienced a number of different systems of rule in the period between the Napoleonic wars of the early 19th century and the outbreak of the Civil War. During most of the 19th century, Spain was a constitutional monarchy, but under attack from various directions. The First Spanish Republic, founded in 1873, was short-lived. A monarchy under Alfonso XIII lasted from 1887 to 1931, but from 1923 was held in place by the military dictatorship of Miguel Primo de Rivera. Following Primo de Rivera's overthrow in 1930, the monarchy was unable to maintain power and the Second Spanish Republic was declared in 1931. This Republic soon came to be led by a coalition of the left and center. A number of controversial reforms were passed, such as the Agrarian Law of 1932, distributing land among poor peasants. Millions of Spaniards had been living in poverty under the firm control of the aristocratic landowners in a quasi-feudal system. These reforms created strong opposition from the landowners and the aristocrats. At the same time, the anticlericalist acts of the government infuriated the clergymen, while military cutbacks and reforms further alienated the military.

On 14 April 1931 the Second Republic was declared in Spain. Centuries of monarchical tradition - interrupted only by the brief interlude of 1873-74 - were abandoned. King Alfonso XIII left the country following local and municipal elections in which Republican candidates won the majority of votes in urban areas. The departure led to a provisional government under Niceto Alcalá Zamora, a Catholic and a landowner from Cordoba, and a constituent Cortes drew up a new constitution, which was adopted on 9 December 1931,[citation needed] after being passed by a referendum three days earlier.[citation needed] The Spanish Constitution of 1931 meant the legal beginning of the Second Spanish Republic, in which the election of both the positions of Head of State and Head of government was meant to be democratic.

The 1931 Constitution was formally effective from 1931 until 1939; however, in the spring of 1936, at the onset of the Spanish Civil War, it was largely abandoned by the Republicans in favor of leftist revolution.