The Reasons for the Republican Defeat in the Spanish Civil War
Last but not least, the single most important factor is the foreign support for the Nationalists. Germany, Italy and Portugal supported the Nationalists despite the less then best efforts of the non-intervention committee. Germany sent cadres and, often equipped with excellent equipment and Hitler provided over 500 aircraft on credit, while Italy brought in 70,000 troops and also provided extensive military aid. Crucial support also came from the Texaco oil company. Petroleum products are crucial to modern mechanized armies and Texaco sent plenty of oil at very good prices with easy credit. By contrast the Republic was backed by the USSR which had an agenda in Spain, and the Mexican government which didn’t have much of anything.
In sum, the Nationalist forces were a well-supplied and coordinated professional force against a splintered amateur force that was ill supplied and poorly organized. The fact that they lasted so long is a tribute to their commitment.