Masterpiece!!! Similar movies???

Atonement is the closest.
Cold Mountain is kind of similar as it's a romantic movie with war as background and beautiful cinematography but is not on the same level artistically speaking.
If you liked the cinematography and the beautiful cinematic ideas the movies contains, then you better check the other movies directed by Jeunet (I hope you saw Amelie with the same leading actress).
If you're into emotional WW1 movies then: Paths of Glory. Also, Joyeux Noel seems to gather some love from moviegoers.

Waterloo Bridge (1931 1940)
Une aussi longue absence (1961)
Les parapluies de Cherbourg (1964)
I girasoli (1970)
Limbo (1972)

This is one of my favorite movies of all time, but Tarsem's, The Fall, is a right up there too and a bit similar in its magical realism.

You might also check out another French movie about the Great War called 'la Chambre des Officiers'' ...

It doesn't have much combat, but the story is completely immersed in the feel and atmosphere of the way some French soldiers experienced the war. It is about soldiers who were wounded in the face and horribly disfigured. In France they were called 'les gueles cassées' - the broken faces.

LCdO is also filmed in a warm glow, reminiscent of Long Dimanche's use of the look of old color autochrome photography. But it's a low budget film compared to Long Dimanche.