My dad was a combat medic in the Pacific in WWII (147th infantry; at Guadalcanal, Iwo Jima, Saipan, and Okinawa - odd that an Army unit was the only US unit to fight in all 4 “Marine” operations.)

Anyhow, somewhere I have an old Army pamphlet published after Guadalcanal that took his unit’s experience on jungle fighting and passed it on to other newly arriving units. In it, there is a section on “what to carry” that sounds oddly like it was written by a backpacker. For eating and drinking, it told soldiers to forget about the mess kit and full set of utensils, and told them to take their canteen, canteen cup (which could also be used as a pot), and a spoon. It told them they wouldn’t need all the clothing the standard manuals said - but advised two or three changes of socks. There were some other tips, but I can’t remember them now, and can’t find the pamphlet (I think my son has it.)

Looking back to the Civil War, the infantry soldier soon learned he didn’t need the entire official kit the Army regulations called for - including the pack. Instead, they laid down the shelter half, laid a folded blanket on top of it, put a small pot or fry pan, fork and/or spoon, metal cup, change of clothing, socks, etc. along the edge of the blanket, then rolled everything up and slung the blanket roll across one shoulder; a cartridge box went across the other.

My guess is the ordinary infantryman has, for ages, looked at the manual and then done what makes common sense.