When Benito Mussolini gained power in Italy he began to develop contacts with right-wing forces in Spain. In March 1934 Mussolini met a group of Spanish politicians and generals in Rome who were opposed to the Second Republic. At the meeting Mussolini promised the group 10,000 rifles, 10,000 hand grenades, 200 machine-guns and a million pesetas in cash in event of a military uprising. On the outbreak of the Spanish Civil War Mussolini failed to keep his promise of immediate aid. After a week of negotiations he agreed to sell the Nationalists twelve Savoia S81 bombers. In September 1936 a Non-Intervention Agreement was drawn-up and signed by 27 countries including Germany, Britain, France, the Soviet Union and Italy.

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.

In December Benito Mussolini began sending large numbers of Black Shirts to Spain. By the end of 1936 there were 3,000 members of the Black Shirts in Spain. They took part in the fighting around Madrid and participated in the fall of Málaga in February 1937. By this time their numbers had increased to 30,000. There were also 20,000 members of the Italian Army fighting in Spain. The Italians also played a prominent role in the offensive at Guadalajara. Mussolini insisted that his forces should be used as a single unit. General Francisco Franco was unhappy about this as he wanted the Italians dispersed among his own Spanish units.

On 8th March over 35,000 Italian soldiers and 81 whippet tanks and a company of machine-gunners, went into action at Guadalajara. The Italians failed to breakthrough on the first day and on the 9th March the Republican Army reinforced the frontline with over 20,000 soldiers. The Republicans held the Nationalist for over a week before launching its own counter-offensive on 18th March. Using its best troops, including the International Brigades, the Republicans were able to force the Italians to retreat.

During the failed offensive at Guadalajara, the Italians had 400 killed, 1,800 wounded and had 500 men taken prisoner. The Italians also lost significant quantities of arms and supplies, including 25 artillery pieces, 10 mortars, 85 machine-guns and 67 trucks. General Francisco Franco blamed the Italians for the Nationalist defeat and banned them from operating again as an independent unit in Spain. He insisted that in future the Italians would have to operate in larger units made up primarily of Spanish troops and commanded by Spanish generals.

In August 1937 Italian submarines began torpedoing ships heading for Republican ports. The governments of Britain and France both made protests at this action and the following month Benito Mussolini brought an end to these attacks on shipping. During the Spanish Civil War Italy sent 80,000 men, of whom almost 6,000 belonged to the Italian Air Force, 45,000 to the army and 29,000 to the fascist militia. Italy also supplied 1,800 cannon, 1,400 mortars, 3,400 machine-guns, 6,800 motor vehicles, 157 tanks, 213 bombers, 44 assault planes and 414 fighters.