Two quick points:
The POUM did have Trotskyists in it, but it was not a Trotskyist organisation. As I said (as I was quoted before):
the POUM was led by Andres Nin, Trotsky's former secretary. Trotsky and Nin broke over the issue of taking the help from Anarchists, rather conservitive Basque nationalists, and some really rather conservative support in to the POUM.
Trotsky favored the "United Front" of proletarian elements and only proletarian elements aimed soley at the destruction of capitalism - fascism, of course, being part of capitalism.
The POUM had a grab-bag of other issues it was courting, not all of which were proletarian.
The Stalinists (I know people object to that term, but it's shorthand - just as when I use "trotskyist") favored a "Popular Front" of alligning themselves with smaller capitalists to oppose fascism.
The May Days were rather important as everything was put to the test - Barcelona had already been under worker control. The POUM, Trots, and anarchists wanted it to remain in worker control. The Stalinists wanted to put it under capitalist control until the fascists were defeated. This was the spark that set off the poweder keg. Eventually the Popular Front began liquidating the POUM soldiers. It should be pointed out that had their positions been reversed, the United Front probably would have eventually turned on the Popular Front as traitors as well.
-TIG
Alis Volat Propriis; Tiocfaidh ár lá; Proletarier Aller Länder, Vereinigt Euch!