He arrogantly believed he “could handle Uncle Joe.” FDR the supreme narcissist got handled instead.
Several able historians have remarked on FDR's egotistical belief that he could smooth-talk anybody into anything. He also had a cabal of Stalinist "influencers" in his administration, right up to Harry Hopkins, as we now know from reading the KGB files in the 1990s. Lastly, he was in no shape to negotiate with Stalin at Tehran, never mind Yalta. Churchill was furious with him, but by then Britain was so deeply in debt to the USA that he had to let FDR run the show.