Hardtack
The quintessential soldier’s food, hardtack has been around for centuries in some form or another. Basically, an unleavened bread (almost a cracker), hardtack has been favored by navies and armies because of how long it will keep for and how it’s relatively easy to pack into a bag and carry.
Soldiers in the U.S. Army still get something like hardtack in their MREs (Meal, Ready to Eat), but it’s called crackers now. Some things never change, do they?