Meaning no offense to Juan Carlos - IMO, he's one of the greatest men of the 20th century for his handling of the 1981 coup attempt.
Allow me to present a fairly simple WI - WI in 1981, when the Guardia Civil had its coup attempt, the Spanish king threw his weight entirely behind it. IOTL, his opposition to the coup was a major cause of its failure. Had he supported it, it would certainly have been "more successful," or at least have lasted for a longer period of time.
The two possibilities here seem to be either that fascism is restored in Spain with a successful military insurrection, or else that a violent civil war ensues and the fascist forces are defeated. I think it likely that regardless of the eventual victor there would have been fighting, perhaps an uprising by the Basques or something along those lines. Does anyone think it possible that foreign nations might have intervened in the fighting? If the anti-fascist forces show too strong a Communist influence, it seems possible that Reagan, with his strong commitment to anti-Communism, might have given some US support to the Spanish government. After all, Spain had been friendly to the USA under Frano. In my opinion, even if the US merely did nothing in regards to the fighting in Spain, its status in the eyes of Europe would likely suffer.
Interesting situations exist, with how the various Western European nations might react to this situation. I can see French and West German teenaged Communists trying to sneak into Spain to help the rebels... while at the same time, France elected a Socialist president in 1981. Admitedly, not until May, but still... it'd be interesting to see how a Socialist government in France would react to a second Spanish Civil War.
And, of course, the way in which the USSR reacts to such a situation would be interesting! Once can imagine a sort of propaganda effort - something along the lines of how it's a sign that socialism's time has come in Western Europe.
Speaking of which, I can imagine terrorist activity alone the lines of the German Red Army Faction occurring in "sympathy" with the Spanish anti-Fascists... it could get messy!
Anyways, what do you think about the possibilities?