On the outbreak of hostilities in Spain, Leon Blum, the prime minister of the Popular Front government in France, initially agreed to send aircraft and artillery to help the Republican Army in the war with the Nationalists. However, after coming under pressure from Stanley Baldwin and Anthony Eden in Britain, and more right-wing members of his own cabinet, he changed his mind.
Baldwin and Blum now called for all countries in Europe not to intervene in the Spanish Civil War. The first meeting of the Non-Intervention Committee met in London on 9th September 1936. Eventually 27 countries including Germany, Britain, France, the Soviet Union, Portugal, Sweden and Italy signed the Non-Intervention Agreement.
Benito Mussolini continued to give aid to General Francisco Franco and his Nationalist forces and during the first three months of the Nonintervention Agreement sent 90 Italian aircraft and refitted the cruiser Canaris, the largest ship owned by the Nationalists.
On 28th November the Italian government signed a secret treaty with the Spanish Nationalists. In return for military aid, the Nationalist agreed to allow Italy to establish bases in Spain if there was a war with France. Over the next three months Mussolini sent to Spain 130 aircraft, 2,500 tons of bombs, 500 cannons, 700 mortars, 12,000 machine-guns, 50 whippet tanks and 3,800 motor vehicles.
Adolf Hitler also continued to give aid to General Francisco Franco and his Nationalist forces but attempted to disguise this by sending the men, planes, tanks, and munitions via Portugal. He also gave permission for the formation of the Condor Legion. The initial force consisted a Bomber Group of three squadrons of Ju-52 bombers; a Fighter Group with three squadrons of He-51 fighters; a Reconnaissance Group with two squadrons of He-99 and He-70 reconnaissance bombers; and a Seaplane Squadron of He-59 and He-60 floatplanes.
The Condor Legion, under the command of General Hugo Sperrle, was an autonomous unit responsible only to Franco. The legion would eventually total nearly 12,000 men. Sperrle demanded higher performance aircraft from Germany and he eventually received the Heinkel He111, Junkers Stuka and the Messerschmitt Bf109. It participated in all the major engagements including Brunete, Teruel, Aragon and Ebro.
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 Non-Intervention 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.