That the aid the Nationalists received (such as the airlift to Seville at the start of the war) was critical for their victory is undisputed. However, outside of that I have read a bit of contradicting reasons to why the Spanish Civil War played out as it did. I have read for example that the disunity on the Republican side is over- and understated, which doesn't really help in getting a clear answer to this question. I was wondering if there was a somewhat consensus here on the reasons for the outcome of the war.
Was it the loss of military leadership? Was Franco a more effective ruler than the Republican side? Was it that the Nationalists had the Army of Africa as the most capable armed force on their side? Infighting on the Republican side? And how do you even effectively quantify the reasons why the Republicans failed to win the civil war even though their starting position was better?
The difficulty here is in the extent that foreign intervention can be straightforwardly isolated from other factors. Republican control over most of the Spanish industrial heartlands, for instance, mattered a great deal less if the war was going to be fought with weapons and supplies that were to a large extent sourced from outside Spain. The interrelatedness of this and other factors means that there is never going to be a definitive, single answer to the question, and different historians will always come to differing conclusions.
In any case, this older answer of mine goes into the question from a few different perspectives.