The basic gist is this: Franco and his minions were fighting to keep Spain as it was-largely a place where wealth was consolidated in the hands of very, very few with no opportunity for the masses to get out of a level of poverty that they had wallowed in for several centuries.

The Republicans were fighting to keep the newly founded Republic alive and maintain open elections and a government that was, at some level, similar to
the US. The sticky part for Americans is the fact that the Republicans largely Communists, Socialists and Anarchists. But many of the ideals they were fighting to preserve were very American in nature. If that same group would have been a little more centrist, the Americans, Brits, French eg al would have probably jumped to Spain’s defense.

Ironically, the US ended up playing nice with a communist dictator 1,000 times worse in Stalin than what they would have worked with in adoquín had they supported the Republic.

Not to go too far afield, but that same fear of communism led to much of what became the Vietnam War.