From Here to Eternity isn't really a war film until the very end of the movie—but that still counts, dagnabbit.
When the attack on Pearl Harbor happens, the course of the movie's action gets totally interrupted. Everyone's been dealing with their own domestic problems—having affairs and knife fights and so on—but now that war is on, things are about to get way different. The nature of all those problems changes.
(Plus, it would be wrong to say that a movie dealing with the outbreak of the American role in the biggest war in the history of the world is somehow not a war film.)
The movie's a romance too, though not a romantic comedy by any means. There are two major love stories in the film, Prewitt's relationship with Alma/"Lorene" and Warden's affair with Karen Holmes. Spoiler Alert: no one ends up happy.
These love affairs are pretty much doomed, and in Alma's case she claims she's not in love with Prewitt. Valentine's Day, it ain't.
Melodramatic in a Good Way
The "romance" part of From Here to Eternity is closely tied to its being a melodrama… which isn't a bad thing. You can poo-poo the term "melodramatic" if you want, but this movie is melodramatic in more of a good way (given its status as a classic and everything).
It's got sensationalistic characters and events designed to give you all the feels: Captain Holmes gets drunk, passes out, and causes his wife to give birth to a stillborn baby! Sergeant Warden is sleeping with Captain Holmes' wife! Sgt. Judson beats Maggio to death, and then Prewitt kills Judson in a knife fight! Pearl Harbor happens and changes the nature of everyone's problems!
We bet your heart rate rose just reading about all that.