Great movie.
Sterling Hayden was a Marine and OSS agent, earning a Silver Star in WWII, he played the part of Crazy base commander in Dr Strangelove.
As an OSS agent, he work with the communist partisans against the Nazies in Yugoslovia and after his return to the states, joined the Communist Party. Which ran him afoul of the McCarthy hearings and put a serious dent in his Hollywood career (though in truth, his passion was travel not acting).
Casting him in that role was one of Stanley Kubrick's ways of giving DC the finger