The ‘Republicans’ in Spain included many factions. Of course, they got support from the USSR. Some of the ‘Republican’ factions in Spain were conducting massacres that were being commemorated as late as 2000. The Nationalists were, of course, supported by Germany. Franco rewarded them by making Spain neutral in WWII. Note that Spain contributed to the German war effort by allowing German bases in Spain. Spain traded with Switzerland during the war, while allowing Allen Dulles to run the European OSS our of Bern.

My sense of it, is that Franco somehow knew that Germany would lose WWII and that he and Spain needed to avoid future enemies. Just a guess.