When fighting began in 1936, after the rebellion of part of the officer core against the Second Spanish Republic, it was soon evident that two factions which would fight each other in the next three years represented among other things international antagonisms which dominated Europe and the rest of the world in the first half of the 20th century, especially in the 1930s era.
Faction remaining loyal to the Republic represented mostly working class and some elements of the middle class, which had most support in urban areas like Madrid, Barcelona and Valencia and in industrial areas, was a diverse mix of politically left-wing elements ranging from anarchists, communists to social-democrats, and it had support of most intellectuals of the time. In the political context of the time while many countries wanted to stay neutral during the conflict, having in mind British and French passive stance towards the rise of Nazism and Fascism in Europe, the only country willing to support the Republican cause was the Soviet Union, led at the time by Joseph Stalin.
On the other side there was also a diverse variety of more conservative elements ranging from religious Catholics and clergy, Carlists, monarchists up to national-syndicalists (known as Falangists) who had many similarities with Nazi party in Germany and Fascists in Italy. It was backed by upper classes in the Spanish Society from agricultural land owners, businessmen to most of the army officers. Heavily influenced by European political context of the time it soon had large support of Nazi Germany and Fascist Italy. During the war they were known as the Nationalists.
First phase – war of the biplanes
Prewar Spanish air force was small and obsolete. Soon, a lot of equipment came from Nazi Germany and Fascist Italy, which meant that Nationalist forces had the upper hand in the air during general Francos initial offensives and bombing raids on the major cities.
Fighter squadrons were equipped with biplanes, usually Heinkel He 51 and Arado Ar 68.
These planes although already out of date faced little or no opposition, and despite their inferiority for the time they ruled the skies over the country, paving the way for initial nationalist offensives.
Besides materiel support Germany and Italy sent numerous volunteers including combat units and advisors, so many of the pilots of these aircraft were Germans. German Condor Legion and Italian Aviazione Legionaria will play important advisory and direct combat role for Nationalist Air Force.
Having air superiority meant that many Spanish major cities, most staying loyal to Republican cause were constantly bombed. Most famous example being the Bombing of Guernica, in which the entire town was raised to the ground by Nationalist bombing raid.
Bombing raids were devastating although most of them were carried out by Junkers Ju-52 trimotor transport planes.
Second phase – the Republic gets equipped
One of the first pieces of equipment delivered by the Soviet Union to Republicans were bombers. Delivered in parts by ships, through the Black Sea and Mediterranean and assembled in Spanish ports, Tupolev SB-2 bombers known in Spain as Katiuskas made an instant impact.
Flown by Soviet pilots, they instantly bombed Nationalist airfields from which bombing runs on Madrid were carried out, destroying all of the enemy aircraft on the ground. Because of actions like these Soviet pilots remained very popular among the Spanish populace till the end of the war.
Nationalist fighters of the time were not an issue for these bombers because they flew at very high speeds, which their adversaries could not match.
Soon after bombers Republicans demanded fighter aircraft from Moscow, which Stalin dispatched after he was payed in gold. Interesting thing was, that because of weapons transfers like these most of the Spanish gold reserves, which were 4th largest in the world at the time, because of the vast Spanish colonial empire it once had, ended up in Moscow. This argument was constantly used by Nationalist propaganda throughout and after the war.
Soon, Republicans were armed with the same fighter aircraft Soviet Air Force was equipped in that period: Polikarpov I-15 and I-16.
These aircraft outclassed Nationalist biplanes in every aspect and soon Republic had air superiority, which it held through most of the 1937. Because of its exterior design Polikarpov I-16 (lower image) known in Spain as Mosca or Rata, was incorrectly mistaken for Boeing P-26. The Republicans got around 475 of these aircraft from USSR during the war.
Despite having air superiority, Republicans were not able to capitalize on that in their ground operations. Most of their bombers flew on too high altitudes, often missing enemy positions. Their fighter aircraft never dived on enemy forces to strafe them with their machine-guns, and overall there was weak coordination and liaison between army ant the air force.
Nationalists, on the other hand, advised by Germans used Blitzkrieg tactics on the ground, utilizing constant coordination with air force, which was able to deliver close-in support to troops on the ground. Overall their air force performed better.
Third and final phase – most modern of the Axis inventory arrives
Having in mind Republican air superiority, and the fact that obsolete aircraft they sent before could not cope against the latest Soviet designs, Germans started to deliver most modern aircraft they had. The aircraft that turned the tide of the war in the air was Messerschmitt Bf-109 fighter, which would earn the reputation of the best aircraft of the entire war, which was not a big surprise because it was the most modern. Having good results in Spain meant that it would be the main German fighter in the first half of WW2. It was first issued to equip the Condor Legion.
From there on Republican bombers suffered heavy loses and their fighters lost air superiority. At the moment the Nationalists did not only had the upper hand in quality but also in numbers, while the Republicans had fewer and fewer available aircraft to counter the threat. Their answer to this was buying more powerful Wright-Cyclone engines from the United States for their Moscas and even up-arming them with additional machine-guns, but it could not prevent the Nationalists from gaining the upper hand.
From there on Germans also tested their latest bombers, where Dornier Do-17, Heinkel 111 and Junkers Ju-86 were dispatched to Spain. Of these aircraft Heinkel 111 proved to be the most efficient and it remained the main work-horse of German bomber forces through the entire WW2.
Among the most successful aircraft sent by Italy to aid Francos cause was the Savoia-Marchetti SM-79 trimotor bomber. Facing relatively small fighter opposition it flew constant bombing missions on major Spanish cities without suffering much losses. This fact prompted many countries including Romania and Kingdom of Yugoslavia to purchase the aircraft. But its luck will change during WW2.
Soviet Union delivered around 800 planes to the Republic during the war. Germany and Italy delivered around 1200 planes to Nationalists, about 600 each. As war went on Nationalist air force got superior in terms of quality and quantity. Use of modern tactics implemented by German advisors from the start and efficient coordination between army and their air force resulted in successful Nationalist offensives through the entire war. Other important fact was that Nationalists had much larger support during the war. Italy and Germany supported Franco from the start and sent enormous materiel, funding, advisors even combat units to assist them. Republicans, mostly because of American, British and French neutrality and passive stance towards the conflict (although many volunteers from these countries fought on the Republican side in the international brigades) had to rely on sole support of the Soviet Union, which to some level supported their cause, but covertly and at a heavy price. Although operating the same tanks and aircraft Soviets used at the time, most of Republican other equipment from small arms to artillery was obsolete, some pieces dating to 1860. Besides this their equipment was scarce. Nationalists on the other hand were well equipped with modern weapons. In their air defenses they used German 88mm anti-aircraft guns which proved very powerful even during WW2.
Spanish Civil War served as a testing ground for German equipment and tactics. Their Blitzkrieg in the first stages of WW2 was extensively tested in Spain. Although many of their modern aircraft performed well during this conflict, faced against heavy fighter opposition later on during the Battle of Britain, their fighters, and especially their bombers would suffer huge losses.