On the other side, you had most of the professional Spanish military, the church, the land owners, big business, and the bourgeoisie.
The Russians supplied and assisted the "Republicans" as the left called themselves. The Germans and Italians assisted the "Nationalists" under General Franco.
It was a war to the knife. Each side shot prisoners as a matter of course, and Thomas thinks the number of people put up against the wall or simply shot in the bull rings or any other convenient place by both sides surpassed 500,000. That doesn't include battlefield causalities, just the liquidation of anyone associated with, or rumored to be associated with, the other side who had the bad luck to fall into the hands of their opponents.
I also read William Manchesters "The Last Lion." It's one of my favorite books. Churchill has always been a man for whom I have great respect. Most books deal with him during that phase of his life when he was Prime Minister during WW2. But people don't know that as a Lieutenant in the British Army, he fought in Afghanistan, the Sudan and South Africa. When I say fought I don't mean he was with the gear in the rear. He was out doing the hand to hand version, in both Afghanistan and Africa. In South Africa he was a correspondent and was involved in combat but that was not his primary concern there.
William Manchester was a Marine in the Pacific War, and he was up front , as an infantryman. He was a great historian in his latter years, but psychologically he was severely damaged by his experiences during WW2 and he never got those issues under control. At the end of his life, he went back to the islands where he served and tried to straighten himself out. When you read his book about that , "Goodbye Darkness" you can tell he wasn't successful. But at least he made the attempt.