what really is the best war movie

I agree with YESBLOOM that the singing was unnecessary. In fact, it interrupted the action and slowed down the story.
It was written by Earl Hawley Robinson, a composer of pro-labor, pro-racial equality songs who was active in Hollywood in the 1940s until being blacklisted for his leftist leanings. He ended his musical career as a high-school music teacher in New York City and died in a car crash in 1991, at the age of 81.
I remember singing his beautiful cantata "The Lonesome Train," about the funeral for Abraham Lincoln, back in the 1950s. You may have heard Frank Sinatra singing Robinson's "The House I Live In" in a 1945 movie whose name I've forgotten. It was a heartfelt song about the United States' ethnic and religious diversity.
In this movie, however, Robinson's contribution falls flat. It seems tacked on and forces what little action there is to pause. Notice that when the song is played, the actors freeze into tableaus. Not that the music is bad (it isn't) but it is misplaced.
Another odd treatment is the narration by Burgess Meredith. It is inconsistent. Starts at the beginning of the movie and peters out halfway through it.
Having said all this, I still consider "A Walk in the Sun" one of my favorite war movies, if not my favorite.