The Spanish Civil War was a major conflict fought in Spain from 17 July 1936 to 1 April 1939. The war began after a pronunciamiento (declaration of opposition) by a group of generals under the leadership of José Sanjurjo against the elected government of the Second Spanish Republic, at the time under the leadership of President Manuel Azaña. The rebel coup was supported by a number of conservative groups including the Spanish Confederation of the Autonomous Right, monarchists such as the religious conservative Carlists, and the Fascist Falange. The uprising was authoritarian and anti-separatist in inspiration; anti-Basque and anti-Catalan feeling marked the Nationalist zone.

Following the only partially successful coup, Spain was left militarily and politically divided. From that moment onwards, general Francisco Franco began a protracted war of attrition against the established government for the control of the country. The rebel forces received the support of Nazi Germany and the Kingdom of Italy, as well as neighbouring Portugal, while the Soviet Union and Mexico intervened in support of the Republican government or loyalist side.

A civil war is a war between organized groups within the same nation state or republic, or, less commonly, between two countries created from a formerly united nation state. The aim of one side may be to take control of the country or a region, to achieve independence for a region, or to change government policies. The term is a calque of the Latin bellum civile which was used to refer to the various civil wars of the Roman Republic in the 1st century BC.

A civil war is a high-intensity conflict, often involving regular armed forces, that is sustained, organized and large-scale. Civil wars may result in large numbers of casualties and the consumption of significant resources.

Civil wars since the end of World War II have lasted on average just over four years, a dramatic rise from the one-and-a-half year average of the 1900-1944 period. While the rate of emergence of new civil wars has been relatively steady since the mid-19th century, the increasing length of those wars resulted in increasing numbers of wars ongoing at any one time. For example, there were no more than five civil wars underway simultaneously in the first half of the 20th century, while over 20 concurrent civil wars were occurring at the end of the Cold War, before a significant decrease as conflicts strongly associated with the superpower rivalry came to an end. Since 1945, civil wars have resulted in the deaths of over 25 million people, as well as the forced displacement of millions more. Civil wars have further resulted in economic collapse; Burma (Myanmar), Uganda and Angola are examples of nations that were considered to have promising futures before being engulfed in civil wars.