I'm not sure if this was collaboration, Trotsky himself openly admitted that he was helping Fascists.
Political Dialogue (1938, Dec 20) - page 155 in pdf-related (couldn't find online):
NB: this is all Trotsky, both asking questions to himself and answering
> But you cannot deny that the fascists use your criticisms. All the reactionaries shout for joy when you unmask the USSR. … You reveal only the defects of the USSR, and consequently this lowers the prestige of the first workers' state in the eyes of the workers of the world, and thus objectively you are helping fascism.
To put things into context, to demonstrate that Trotsky was not vaguely "criticising" USSR, this is his position at a month before (i.e. at the end of 1938, less than a year before WWII). Notable points:
a) there is a need to start Civil War within the USSR
b) Proletariat should not resist to Fascist invasion
A Fresh Lesson On the Character of the Coming War (1938, Oct 10):
a) there is a need to start Civil War in the USSR
b) Proletariat should not resist to Fascist invasions
In case there is some doubt, I'll present Trotsky's logic.
Defending oneself from Nazi invasion is wrong, since you might accidentally become imperialist yourself:
Defeat is okay, because it is temporary. One must obediently march into the death camps - anything else is doubting the Revolution.
> This does not exclude that in passing the imperialist war may improve or worsen the position of this or that “nation”; or, more exactly, of one nation at the expense of another. … In reality all speculative arguments of this sort and raising bogies of impending national calamities for the sake of supporting this or that imperialist bourgeoisie flow from the tacit rejection of the revolutionary perspective and a revolutionary policy'.
And even if Trotskyite Fourth International does not defeat Fascism (after Trots defeat USSR in an armed uprising) and all "revolutionaries" die in death camps, it's okay because new generation can look upon them and say "fucking morons".