The Santoa Agreement, or Pact of Santoa, was an agreement signed in the town of Guriezo, near Santoa, Cantabria, on 24 August 1937, during the Spanish Civil War, between politicians close to the Basque Nationalist Party (Spanish: Partido Nacionalista Vasco - PNV), fighting for the Spanish Republicans, and Italian forces, fighting for Francisco Franco.

During the Battle of Santander, the Francoist troops pierced through the Republican defense lines. The Republican troops, including the Basque army, retreated to the west in a disorderly fashion, with numerous desertions.After the fall of Bilbao, almost all of Basque territory had fallen into Franco's hands. Juan de Ajuriaguerra, the president of the Biscay Regional Council of the PNV, negotiated a surrender agreement with the Italian army command. The PNV offered to surrender the Basque army in exchange of its prisoners being treated as prisoners-of-war under Italian command and PNV members being allowed to go to exile on British ships.

The Basque nationalist units of the Republican army in the Basque territory, fighting under the direction of Basque President Jos Antonio Aguirre, met at Santoa and surrendered to the Italian forces on 24 August. When news of the agreement arrived to his headquarters, Franco cancelled the agreement and ordered the immediate jailing of the 22,000 captured soldiers in Santoa's El Dueso Prison. Three months later, around half of them had been freed, and the other half remained in prison, and 510 were sentenced to death, a smaller reprisal proportion than registered elsewhere. Ajuriaguerra, the highest rank in the PNV, was released from prison in 1943.

Because the agreement was carried out by the PNV behind the Republican government's back, the agreement is also known as the Santoa Treason.

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.