I found an army site about the 1898 war.  Https://www.spanamwar.com/foodarmyrecipes.html   interesting that in the nearly 40 years since the Civil War, the Government supplied food for troops hadn't changed one jot. These descriptions could be word for word from letters written in the Petersberg Lines, or at Chatanooga.

-BEL.

Below are some recipes derived from U.S. Army first-hand accounts. The goal is to include accounts that provide enough detail to allow for the recipe to be recreated. More recipes will be added as they are found.

Coffee:

"Thanks to a beneficent government we had coffee about all of the time, if we had nothing else. It came to us in the berry, in paper packages, and our chief concern as to coffee was how to grind or pulverize it. Usually this was done by the simple but slow process of putting a few berries in our tin cups and pounding them with a stick or tent pole until they were broken enough to steep. Then the cup was filled with water and placed in the fire until the coffee boiled when the cup was taken out by means of a cleft stick or a bayonet and laid aside cool sufficienty to drink. There was plenty of barbed wire everywhere and by means of the wire cutters rude grates were made on which the tin cup was placed." [Account from the 2nd Massachusetts Volunteer Infantry as encamped before Santiago before the city surrendered].

"Sludge"

"Sometimes when we had canned tomatoes we made 'sludge,' a simple confection of tomatoes and broken hardtack, with, at times, a few 'strings' of corned beef thrown in to give it, not taste, but more body." The context indicates that it was cooked in a manner similar to the coffee, above. [Account from the 2nd Massachusetts Volunteer Infantry as encamped before Santiago before the city surrendered].
 

"Santiago Sludge Cakes"

"Then we made 'Santiago sludge cakes,' composed of pulverized hardtack and water, the mixture being patted into cakes and fried in bacon grease. Sometimes a bit of sugar was sprinkled over them, and we deluded ourselves into the belief that we were eating something very fine. Another method of preparing this delicacy was to mix in some canned tomato." [Account from the 2nd Massachusetts Volunteer Infantry as encamped before Santiago before the city surrendered].

The same sore of dish was recorded by one of Roosevelt's Rough Riders in a letter home, though did not name the confection. Rough Rider Kirk McCurdy wrote to his father on June 28, 1898 and provided the following description. McCurdy did actually seem to like the resulting meal:

"We...soak about four hardtack in water until it is dough, add salt, then mix in coffee, fry in bacon grease, put a little sugar on top and enjoy it to its full extent."