Juan Negrín in the context of Republican faction (Spanish Civil War)

The phrase Spanish Republic at war refers to the Second Spanish Republic during the Civil War of 1936–1939. It represents the last stage in this war's history. The territory that remained under its control after the coup of July 1936—which was called the republican zone or the loyalist zone—gradually reduced as the rebel zone spread, until it was finally occupied in its entirety by the Francoist side (which had referred to the areas as the red zone throughout the civil war).

During this time, there were three consecutive governments: the one presided over by José Giral, from Republican Left, although during his short term (July–September 1936), real power was actually in the hands of the hundreds of committees that were created when the social revolution of 1936 broke out. The next government was presided over by Francisco Largo Caballero, the leader of the Spanish Socialist Workers' Party (PSOE) and the Unión General de Trabajadores (UGT), one of the two trade unions that had led the revolution. Finally, the third government was presided over by Juan Negrín, also from the PSOE, as a consequence of the fall of Largo Caballero's government after the events of May 1937. Negrín governed until early March 1939, when Colonel Segismundo Casado's coup took place, putting an end to the Republican resistance and giving way to the victory of the rebel faction led by General Francisco Franco.

The International Brigades (Spanish: Brigadas Internacionales) were volunteer soldiers organized by the Communist International to assist the Popular Front government of the Second Spanish Republic during the Spanish Civil War. The International Brigades existed for two years, from 1936 until 1938. It is estimated that during the entire war, there were some 32,000 Brigaders, yet at no single moment were there more than 18,000 actually deployed. Beyond the Spanish Civil War, "International Brigades" is also sometimes used interchangeably with the term foreign legion in reference to military units comprising foreigners who volunteer to fight in the military of another state, often in times of war.

The headquarters of the brigade was located at the Gran Hotel, Albacete, Castilla-La Mancha. They participated in the battles of Madrid, Jarama, Guadalajara, Brunete, Belchite, Teruel, Aragon, and the Ebro. Most of these ended in defeat. For the last year of its existence, the International Brigades were integrated into the Spanish Republican Army as part of the Spanish Foreign Legion. The organisation was dissolved on 23 September 1938 by Spanish Prime Minister Juan Negrín in a vain attempt to get more support from the liberal democracies on the Non-Intervention Committee.

Spanish coup of March 1939, in historiography often referred to as Casado's coup (Spanish: Golpe de Casado), was a coup d'état organized in the Republican zone against the government of Juan Negrín. It was carried out by the military with support of the Anarchists and a major faction of the Socialists; its leader was commander of the Army of the Centre, Segismundo Casado. The conspirators viewed the Negrín government as a hardly veiled Communist dictatorship. Most concluded that the government-endorsed strategy of unyielding resistance against the Nationalists would produce nothing but further deaths and sufferings while the war had already been lost. The military and some politicians intended the coup as a first step towards opening peace negotiations with the Nationalists; for the Anarchists and Socialists the priority was to remove the Communists from power.

The coup began on March 5, when rebels declared the setup of their own quasi-government, Consejo Nacional de Defensa (CND), based in Madrid. Following few hours of discussions the Negrín government, at the time based near the town of Elda at the Levantine coast, decided that any resistance was pointless; the following day they left Spain. The rebels easily took control of all provincial capitals with very little or no fighting recorded. The exception was Madrid, where the local Communist executive, unaware of the PCE Political Bureau decision to avoid bloodshed, mounted resistance. Communist-controlled units recalled from the frontline took control of most of Madrid and on March 9 they were closing in on CND headquarters. However, Anarchist-controlled units also recalled from the frontline entered the city on March 10 and in 3 days they ensured the triumph of the rebels. During the next 3 weeks the Republican zone was ruled by the CND.