The political situation in western Europe in the 1930s was explosive, specially so in Spain. On the one hand there was the then quite new Soviet Union stoking unrest in a, by contemporary standards, shockingly poor, unhealthy and exploited working and peasant class. On the other hand, you had much of the rest of society scared out of their wits by the very real prospect of revolution at their doorstep, and more than willing to support and make do with a strongman that, first and foremost, promised to crush the communists. Either side would have crushed the other if victorious.
I recommend anyone to read good books about the Spanish Civil War (like for instance Hugh Thomas' very readable history) to understand the huge polarization of both extremes, the violence, and how those in the middle where totally swept aside. I doubt that the only possible outcome, given those conditions, could have been but one side exterminating the other, either the left or the right.
Our world is, fortunately, very different. Of late, given an increasing political polarization and rise of populism, comparisons have been made with the 1930s, but if you start investigating you will realize that current conditions are nowhere near as bad.
I don't know where you are from, and how familiar you are with European politics, but modern euro-socialism is much closer to pure capitalism than anything the stalinists would have created in Spain.
I strongly dislike Franco, and I am aware that thanking him for preventing a stalinist Spain may turn heads these days. But the fact is that this is exactly what the West thought at the time. Franco became a pariah right after World War II as the only remaining leader that had supported Hitler. However, as soon as the reality of the Cold War kicked it, all that was forgotten, and western leaders (and specially the USA) started toasting him, indeed, as the man who stopped communism in a country in a very strategic position.
I hardly think there was a unified Western view at the time (perhaps amongst governments but not individuals). George Orwell fought in Spain against Franco's troops in an anarchist unit (POUM) and he hated Stalinism as strongly as anyone.
His Homage to Catalonia is a superb account of his time in Spain (and just how chaotic things were politically):
Sure, I meant across decision makers. Realpolitik (the rest is propaganda).
Before and in the beginning of the Spanish Civil War, when the great Orwell fought, the left in Spain was extremely atomized: there were the anarchist (very active in the beginning), the socialists, the communists (actually a minority initially), and many splinter groups (like POUM). But, just like an equally varied right swiftly unified under Franco, so did the left, more slowly, under the "oficial" communist faction, which, although it decided to be mostly pragmatic until the war had been won, fought some of the other leftists as fiercely as it fought the right.
Yes, that was one factor for sure. But it was actually really complicated, quite mind boggling in fact. The thing is that, in spite of Stalin being pretty much the only effective external support the Spanish republic had, the communists inside Spain were pretty much a marginal group when the war started, and had to resort to all kinds of intrigues to get support from both the population and the, still, official government, and also to had get rid of rival groups, like the ones you mentioned.
Yeah - I read Homage to Catalonia again recently - I have read The Battle for Spain by Antony Beevor a while back but as you say, it is quite complex - I need to read it again!
Or maybe it would be a flop and something closer to present euro-socialism which Spain ended up implementing anyway.
One should avoid being a brutal dictator since you never know at which rate you exchange real blood to imaginary one.