These two.... Churchill was the wittiest head of state ever. Stalin was the ultimate straight man. Devoid of anything like a sense of humor, Uncle Joe never understood a joke and figured whatever it was, he was the butt of it.
The only reason he's smiling here is that the KGB had just informed them that Hitler was suffering the result of eating some bad Mexican at a Berchtesgaden soirée. It seems that the food taster didn't get sick until six hours later while playing, "ride the little horsey" in Göring's quarters.
