if the Republican faction had won the Spanish Civil war, what would the effect have been on World War 2?What would have happened if they had joined the Allies in 1939, , and conversely, what would have happened if the had not?
That would probably be classified as a royal pain for the allies. Assuming the war goes as OTL otherwise that means you have a very weak Spanish state that the allies are stuck propping up and is unlikely to be able to stop Hitler from continuing his blitzkrieg through it. While that probably sees the Spanish people reenact the Guerilla against Napoleon, that still leaves Germany controlling even more of the Atlantic seaboard and worst of all it leaves Gibraltar and the straights hideously vulnerable. Its not clear how easy it would be to defend Gibraltar from a German land attack and if they take Gibraltar the German's can close the Med to the allies completely.
Das Boot graphically illustrates how hard it was for the German's to break in past Gibraltar but they did have access to the Med through the French and Italian coasts (this also kept the Italian fleet bottled up for the duration of the war). If Germany holds Gibraltar then it will be all but impossible to hold North Africa and while that is something of a side show losing it won't do allied moral any good.
On the plus side Germany will be stretched even thinner than it was OTL and without the need to send troops to Africa and later Italy the Allies may be in a position to attack Normandy a year or so earlier. On the other hand without the side shows Germany can concentrate its troops on France more and that also leaves a massive area to be cleaned out and liberated after France and Germany fall, which would also leave an opening for German fanatics to fight on and Italy probably stays fascist and in the fight without the invasion.
In someways the outcome of the civil war was probably the best outcome the allies were going to get in 1939, Franco was a fascist and personally grateful to Hitler but he was also a realist. He knew the civil war had left his country in no fit state for another fight either in 1939 or for about twenty years afterwards. As such while paying lip-service to Hitler he did all he could to keep Spain out of the war by making impossible demands in exchange for his assistance in 1940. He tried to keep Hitler sweet by allowing things like the Blue division while keeping his options open.
As such he left Gibraltar alone and when it became clear that Hitler was finished he did his best to keep the Allies from his door and by that time the Allies had so much on their plate that
doing something about Spain wasn't an option so instead it was economically isolated until it became clear the west needed Franco against the Soviet's. The rest is history...