Answer:
nutritious hardtack
Explanation:
The "nutritious hardtack" was commonly given as a ration for soldiers in the wartime. It's a considered as survival food because it can withstand different conditions in the environment. It mainly consisted of three ingredients (flour, salt and water). During the Civil War, proper storage of food was not yet common, thus, it wasn't easy to provide daily rations to the soldiers. The nutritious hardtack became a solution. However, it was very hard that it could sometimes break the soldiers' teeth. So what they did was to break it first into pieces before biting into it or to make some of the pieces as ingredients in soups.
