I gave an overview of the question of why the Nationalists won here, and more recently in a discussion here got a bit more into the more specific question of how inevitable this outcome was. Any coup attempt is precarious - all it takes is a few key people getting cold feet, or the wrong people refusing their orders, and it can all fall apart, and the coup that started the Spanish Civil War was far from decisively successful. It definitely isn't out of the realm of possibility that the Nationalists might have been defeated early on, and people certainly believed a Republican victory or at least a stalemate was plausible for much of the war. March 1938 was probably the point at which such an outcome would have seemed increasingly implausible, and even then, with both sides' reliance on international support, a sudden diplomatic shift might still have mattered.