Short: Into the Valley by John Hersey, 1943? (a reprint from 1980 or so, and the book is not at hand).
I weeded this from the library collection within the last couple years. I set the book aside to read later and brought it to Scout Camp. Hersey wrote this after working as a war correspondent for Time/Life in the Pacific.
Hersey was eager to see combat and joined a Marine weapons company on Guadalcanal. The weapons company was involved in an advance against the Japanese. After crossing through mountain terrain and criss-crossing a meandering river the company was caught by Japanese snipers and machine guns in a narrow valley. The unit retreated with several casualties. Hersey was with the unit the whole time and was injured himself with broken ribs.
Hersey was sent back to the U.S. to recuperate, visited the New York family of the weapons company commander, and wrote this piece for his magazine.
This brief at about 120 pages and a good read. Hersey's introduction explaining the background of the story is almost as long as the article itself.
Anyhoo. It's Hersey's view of combat and his discussions with the Marines on the ground. Hersey's intro suggest that one Marines response of "apple pie" to the question "What are you fighting for?" started the modern saying. Hersey's question made the men think of what they missed from home. There plenty of other answers by the Marines - many of them written down by Hersey - but the apple pie quote stuck.
The story is also a reminder of how so many of battles, gunfights, shellings, and skirmishes are forgotten or never known. The attack Hersey witnessed was a failure. The weapons company was maneuvering around with several other companies against the Japanese and nothing came of their action. Most of the Marines - and Hersey - never saw the Japanese who were trying to kill them.
Heck, Hersey repeats what you'll read in so many other places: the terrain, weather, and disease were more dangerous than the Japanese.
1. Hersey mentions that this piece had a lasting impact - and repeated repubbing - because it was a predecessor of New Journalism.
2. Ugh. I am not feeling well. I wish I could go to bed.