In the early 1930s Joseph Stalin was deeply concerned about the spread of fascism in Europe. To counteract the growing power of Adolf Hitler and Benito Mussolini, he encouraged the formation left wing coalitions. This resulted in the Popular Front government being formed in February 1936. This was followed by Popular Front government in France in May 1936. In September 1936 a Non-Intervention Agreement was drawn-up and signed by 27 countries including Germany, Britain, France, the Soviet Union and Italy.

Joseph Stalin now became concerned that the Nationalists would defeat the Republicans in Spain. He took the view that four extreme right-wing governments in Europe would pose a serious threat to the security of the Soviet Union. Although Stalin continued to support the idea of the Nonintervention Agreement, he was now willing to supply the necessary military aid to stop a fascist regime being established in Spain.

Stalin encouraged the Comintern to organise the formation of International Brigades. He also sent Alexander Orlov of the NKVD to advise the Popular Front government. Orlov supervised a large-scale guerrilla operation behind Nationalist lines. He later claimed that around 14,000 people had been trained for this work by 1938. The Soviet Union provided considerable help to the Spanish Communist Party to improve its position in the Popular Front government. This included the removal of the socialist Francisco Largo Caballero as prime minister and replacing him with the communist sympathizer, Juan Negrin.

Alexander Orlov also used NKVD agents to deal with left-wing opponents of the Communists in Republican held areas. This included the arrest and execution of leaders of the Worker's Party (POUM), National Confederation of Trabajo (CNT) and the Federación Anarquista Ibérica (FAI).

The Soviet Union were the main suppliers of military aid to the Republican Army. This included 1,000 aircraft, 900 tanks, 1,500 artillery pieces, 300 armoured cars, 15,000 machine-guns, 30,000 automatic firearms, 30,000 mortars, 500,000 riles and 30,000 tons of ammunition.

The Soviets expected the Republicans to pay for these military supplies in gold. On the outbreak of the war Spain had the world's fourth largest reserves of gold. During the war approximately $500 million, or two-thirds of Spain's gold reserves, were shipped to the Soviet Union.