Destroying the revolution? Or failing to restore the Republic? Mostly they were done in by the one-sided arms embargo. As you know, Spain had plenty of money, but no-one but the Russians would sell them weapons, whereas Franco's forces were well-supplied with German and Italian arms and materiel, and even suported by aerial bombardment. This reminds me of Nathan's fantasy that we could have stopped the current war if we'd been more consistently on message.
One Abraham Lincoln Brigade vet here says near as they can tell about 60% of the U.S. volunteers were members of the CPUSA. The International Brigades as a whole, of course, came from all over, including Latin America and Europe, and included a lot of people who had dealt with fascism first hand (the Italian and German brigades most prominently). I suppose you could say that they were 'Stalinist' in the same sense that Russians who fought the Germans to a standstill a few years later were 'Stalinist.' Not sure why you'd want to use the term there, though.
Jenny Brown