POUM ?

Hello, comrades I'm researching about Spanish Civil War and I must confess that all I found resources are Trotkyist or anarchist. I'd like to hear your resources about it.

I would be happy If you answers my questions:

Why Spanish Civil War lost? Why POUM didn't come together with communist party? Is Stalin really sold the revolution to Western powers? Could communists prevail in the war?

Thanks.