Douglas' direct commander is played by Burt Lancaster, whom the President doesn't trust.
Burt is really unlikable in this film.
Ava Gardner has a small part as Burt's former lover who might hold some secrets to help uncover the plot.
Frederic March plays the President, and he makes a really good one, too; I wished his character was our President today.
It's a really good fictional thriller (though based on some real events). I really liked it. Directed by John Frankenheimer (also directed The Manchurian Candidate).
The film was released a few months after President Kennedy's assassination. For audiences at that time, it must have a bit frightening to see such a politically-charged film so soon afterward.
Jacqueline of Another Old Movie Blog discussed this film in an excellent 2016 post here, noting how it remains relevant today.
Bill Crider also wrote about this movie, having remembered the original novel of which it was based, and another good post about this film from Movie Magg.