Left Unity is our motto, but when unity is put into practice it can be extremely messy. The Left in the civil war was made up of many revolutionary and popular organisations, which cooperated to an extent but were divided by their divergent goals and tactics.
What are your answers for finding ways for multiple left factions to act with unity in times of crisis?
the best time to stop Franco and the other fascists was before the war started, it was so fucking clear what was going to happen. maybe having a bunch of conspiratorial fascist generals in charge of your armed forces, is actually a really bad idea. they had already seen with Primo De Rivera that the fear of the left would cause the reactionaries to act, they had so much time between 1931 and 1936 to get their shit in gear for reactionary counter revolution to the Republic, and blew it too fast on the 1934 revolution. CEDA was clearly a huge threat that showed the reactionaries had an organised front already, and had been forced out of government they had legally won, not that this was a bad thing but you should just expect their obvious reaction though when you remove the parliamentary angle for them to get into power. purge them all in one swift move rather than just Calvo Sotelo.
if the war goes ahead though, an actual government of national unity made of the elements of the socialist republican left, anarchists, and Marxists to promote cohesion and a more unified front against fascism. providing an alternative to the reactionary church, when you demand people fight their faith you're gonna lose a lot of political allies, a large effort to push a left wing catholicism would never have hurt. purge disloyal republican generals immediately, and ask the soviet union to help you build a new command structure, though I know the anarchists would not have liked that and it would likely have caused tension and disunity.
I think one of the major issues for left unity breaking down is trust, building organs of a dictatorship of the proletariat that truly includes multiples competing factions shows they buy into actual unity than several disunited factions with friction between them. that's not easy to bring about and requires a lot of compromise.
on the military end of things, I mean obvious linking up the republican northern pocket with the rest of the republic and not allowing the fascists to link up is just pretty obvious. defending Aragon was absolutely vital, digging in there and providing the troops there with supplies to fight in the cold could have slowed down the nationalist advance. and the ultimate take is ofc simply just have your men not lose the fight and win