For Whom the Bell Tolls was a romantic movie filmed on Earth in the year 1943. The movie featured Gary Cooper as Robert Jordan and Ingrid Bergman as María.
This film was found in the motion picture library aboard the Enterprise in 2151 and was the main feature around the time when the Enterprise visited Valakis. The movie brought tears to Commander Trip Tucker's eyes, though he denied the film as being the cause. (ENT: "Dear Doctor")
The name of this particular movie wasn't provided in the final draft script of "Dear Doctor", which was clearly written before For Whom the Bell Tolls was selected.
For Whom the Bell Tolls was based on a novel by the author Ernest Hemingway and was produced by Paramount Pictures. This classic film was about the "tragic love story of an American national fighting in the civil war of a European nation." Furthermore, the Star Trek Encyclopedia, 4th ed., vol. 1, p. 279 noted that "some of the production offices for Star Trek: Enterprise (and several other Star Trek productions) were located in the Gary Cooper building on the Paramount studio lot."