The Catalonia Offensive (Catalan: Ofensiva de Catalunya, Spanish: Ofensiva de Catalua) was part of the Spanish Civil War. The Nationalist Army started the offensive on 23 December 1938 and rapidly conquered Republican-held Catalonia with Barcelona (the Republic's capital city from October 1937). Barcelona was captured on 26 January 1939. The Republican government headed for the French border. Thousands of people fleeing the Nationalists also crossed the frontier in the following month, to be placed in internment camps. Franco closed the border with France by 10 February 1939.

The Spanish Civil War (Spanish: Guerra Civil Española) was a civil war in Spain fought from 1936 to 1939. Republicans loyal to the left-leaning Popular Front government of the unstable Second Spanish Republic, in alliance with both communist and syndicalist anarchists, fought against an insurrection by the Nationalists, an alliance of Falangists, monarchists, conservatives and traditionalists, led by a military junta among whom General Francisco Franco quickly achieved a preponderant role. Due to the international political climate at the time, the war had many facets and was variously viewed as class struggle, a religious struggle, a struggle between dictatorship and republican democracy, between revolution and counterrevolution, and between fascism and communism. According to Claude Bowers, U.S. ambassador to Spain during the war, it was the "dress rehearsal" for World War II. The Nationalists won the war, which ended in early 1939, and ruled Spain until Franco's death in November 1975.

The war began after a pronunciamiento (a declaration of military opposition, of revolt) against the Republican government by a group of generals of the Spanish Republican Armed Forces, with General Emilio Mola as the primary planner and leader and having General José Sanjurjo as a figurehead. The government at the time was a coalition of Republicans, supported in the Cortes by communist and socialist parties, under the leadership of centre-left President Manuel Azaña. The Nationalist group was supported by a number of conservative groups, including CEDA, monarchists, including both the opposing Alfonsists and the religious conservative Carlists, and the Falange Española de las JONS, a fascist political party. After the deaths of Sanjurjo, Emilio Mola and Manuel Goded Llopis, Franco emerged as the remaining leader of the Nationalist side.

The coup was supported by military units in Morocco, Pamplona, Burgos, Zaragoza, Valladolid, Cádiz, Córdoba, and Seville. However, rebelling units in almost all important cities—such as Madrid, Barcelona, Valencia, Bilbao, and Málaga—did not gain control, and those cities remained under the control of the government. This left Spain militarily and politically divided. The Nationalists and the Republican government fought for control of the country. The Nationalist forces received munitions, soldiers, and air support from Fascist Italy and Nazi Germany, while the Republican side received support from the Soviet Union and Mexico. Other countries, such as the United Kingdom, the French Third Republic, and the United States, continued to recognise the Republican government but followed an official policy of non-intervention. Despite this policy, tens of thousands of citizens from non-interventionist countries directly participated in the conflict. They fought mostly in the pro-Republican International Brigades, which also included several thousand exiles from pro-Nationalist regimes.

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. After much of Catalonia was captured in 1938 and 1939, and Madrid cut off from Barcelona, the Republican military position became hopeless. Following the fall without resistance of Barcelona in January 1939, the Francoist regime was recognised by France and the United Kingdom in February 1939. On 5 March 1939, Colonel Segismundo Casado led a military coup against the Republican government. Following internal conflict between Republican factions in Madrid in the same month, Franco entered the capital and declared victory on 1 April 1939. Hundreds of thousands of Spaniards fled to refugee camps in southern France. Those associated with the losing Republicans who stayed were persecuted by the victorious Nationalists. Franco established a dictatorship in which all right-wing parties were 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 that occurred, on both sides. Organised purges occurred in territory captured by Franco's forces so they could consolidate their future regime. Mass executions on a lesser scale also took place in areas controlled by the Republicans, with the participation of local authorities varying from location to location.