Yeah, the letter Castro read which ultimately cinched the split between Che and Fidel was cooked up by the two prior to Che's departure into the Congo (correct me here if I'm wrong). It was designed to give Castro plausible deniability of knowledge of Che's activities both there and in Bolivia in the event the latter was captured. This was done to prevent the alienation of the Soviet Union (Castro probably saw an alliance with the USSR as essential for the reception of foriegn aid) and spurring the U.S. into another invasion attempt.
O, why should wrath be mute, and fury dumb?
I am no baby, I, that with base prayers
I should repent the evils I have done:
Ten thousand worse than ever yet I did
Would I perform, if I might have my will;
If one good deed in all my life I did,
I do repent it from my very soul.
Act V, Scene III; Titus Andronicus--W. Shakespeare