The Spanish Civil War (1936 - 1939) was the result of complex political differences between the Republicans — supporters of the government of the day, the Second Spanish Republic, mostly subscribing to electoral democracy and ranging from centrists to those advocating revolutionary change, with a primarily urban power base — and the Nationalists, who rebelled against that government: these had a primarily rural and more conservative power base.

The war took place between July 1936 and April 1939 (although the political situation had already been violent for several years before) and ended in the defeat of the Republicans, resulting in the fascist dictatorship of Francisco Franco. The number of casualties is disputed; estimates generally suggest that between 500,000 and 1,000,000 people were killed. Many of these deaths, however, were results not of military fighting, but were the outcome of brutal mass executions perpetrated by both sides. Many Spanish intellectuals and artists (including many of the Spanish Generation of 1927) were either killed or forced into exile; also thousands of priests and religious people (including several Bishops) were killed; the more military-inclined often found fame and fortune. The Spanish economy needed decades to recover (see Spanish miracle).

The political and emotional repercussions of the war reverberated far beyond the boundaries of Spain and sparked passion among international intellectual and political communities. Republican sympathizers proclaimed it as a struggle between "tyranny and democracy", or "fascism and liberty". Franco’s supporters, on the other hand, viewed it as a battle between the "red hordes" (of communism and anarchism) and "civilization". However, these dichotomies were inevitably over-simplifications: both sides had varied, and often conflicting, ideologies within their ranks.

The military tactics of the war foreshadowed many of the actions of World War II.