Churchill 'collaborated' with and praised Stalin during WWII because he found that to be in the best interests of England at the time.  So did FDR.

When Churchill was called on it by his political opponents, his response was:

"He is a man of massive outstanding personality, suited to the sombre and stormy times in which his life has been cast; a man of inexhaustible courage and will-power, and a man direct and even blunt in speech"


And, more famously:

"If Hitler invaded hell I would make at least a favourable reference to the devil in the House of Commons."

After the War, England joined with the US and the other NATO members to oppose Stalin, of course.

"Politics makes strange bedfellows."  is an old saying  probably traceable back to Shakespeare's "The Tempest".  

Most Nuclear Powers today tend to say fairly nice things about each other while, at the same time, preparing war plans if that, in the opinion of their leaders, becomes necessary or convenient.