I did study it and was lucky enough to meet a veteran when I was 18. The feeling in free Europe at the time was that Franco/fascism was wrong. Dictator de Rivera had been brought down in 1930 and they had a nascent democracy, so Franco, backed by the other fascist regimes of Germany and Italy, was seen as a bully boy, potentially a threat. The British establishment did not get involved in the conflict, officially, but allowed people to freely go and serve, allowed weapons to be shipped to the Republicans, and took in large numbers of Spanish children.
Remember who was on the side of Franco and what British people in general, especially WWI veterans, might have thought about that fact. We won't ever know whether Tolkien had the phrase forefront in his mind for Gandalf, but given that he could have chosen from dozens, even hundreds of other phrases (I bet he had a thesaurus
), and chose something that famous...it's not to be dismissed. He'll have known what it meant and he didn't dismiss it. Put it this way, it was as well known as modern catch phrases like "We're All In This Together" or "Yes We Can!" and I'd certainly notice if I slipped one of those into a big moment in a story I was writing (and then go and grab a monster pot of Tippex if it was the former
). I didn't realise how well known it was until recently, which is what prompted me to resurrect Rune's thread.
Out of interest, one of Tolkien's former students, and one who held him in great esteem, joined the International Brigades - WH Auden.