@PotomacBobThe US Army has probably the best record of using Nutritionists, technical and industrial "cutting edge" advances and science to develop military rations since the late 1930's.
The "C" Ration (does not stand for canned). "A-Rats" A Rations is food purchased off the market (usually local to the base; some preserved ingredients are procured by the QM (Quartermaster Corps)) and prepared in the Mess Hall or Field Kitchens. B Rations are preserved foods in larger containers for some 20 to 30 men (they are probably the least well known; many people who eat them, and even some of the low ranking Mess personnel may not realize the difference between A and B Rations); A Rations are the Preferred foods to be consumed.
C Rations were the individual field Ration. Because of weight and difficulties supplying several individual meals to each soldier in the field, and particularly the Front Line (trenches in WWI). Mostly A (or B) Rations are what troops ate through the end of the First World War. "Iron Rations" was a common term for preserved Rations issued to individual soldiers from the 1700's, improved by rewards offered by Napoleon. "Iron Rations" were used by the US Army through WWI. Germany, France, Russia, and most other nations issued "Iron Rations" all through WWII, and sometimes beyond.
Iron Rations were often tightly controlled. Soldiers were not supposed to eat them unless Ordered to by a Regimental or higher officer... under pain of Death! Sometimes, Battalion or Company officers would allow the consumption of Iron Rations. Soldiers being soldiers did sometimes eat them without authorization (or need). Rarely was anyone Executed for eating them. Caning, whipping, other beatings, and monetary fines, and other punishments were imposed.
Lack of Iron Rations would hamper the abilities of a unit to force march, or fight for several days without supply.
LOGISTICS LOGISTICS LOGISTICS !!! Until World War Two most armies relied upon horse, or mules, or oxen, to transport supplies. Roads, at least paved roads allowing two way travel, were rare or non existent. Preserving food was time consuming (although not much more expensive than purchasing food for an army).
In 1914 a German or French (and most other nations) Korps/Corps had 50,000 men, requiring Half a Million Iron Rations to feed them for 3 Days. More than 20 German, a Dozen French, a a couple British Corps marched beyond their supply systems abilities to function for 40 days, in Belgium, at the beginning of "The Great War" aka WWI.
Napoleon's dictum is: "An Army Marches on its STOMACH!"
C Rations were developed in the late 1930's, and refined several times during the Second World War. C Rations were issued until c.1984, although the MRE was beginning to be issued in the early 1980's. I have heard some C Rations turned up in the early 1990's, including during Desert Shield
C Rations, though lighter than many other nations field rations, with multiple cans, and other packaging, were disliked by guys who had to carry multiple days of meals. 3 days meals was 9 C Rations.
Long range patrols would open all of their issued rations, and pack only the "essentials"; one technique was to put an intended meal into a clean sock (to prevent the cans from making noise; also, patrols would try to change socks and eat meals twice a day).
Of course, this meant soldiers who were burning a lot of calories were NOT getting optimal nutrition, nor sufficient calories.
The "LURP Ration" (Long Range Recon Patrol) used high tech dehydrated meals. They had lots of advantages, however, they required at least a quart of water, and time to prepare. (They could be eaten dry, kinda.) I heard a lot about LURP Rations from the veteran NCOs in my Recon unit; some of whom had been LURPs in Vietnam (or in West Germany in the 70's). I, other soldiers in my unit had a few opportunities to eat LURP Rations. As a generalization, they tasted better than MREs, but were more of a hassle to prepare, and a bit bulkier.
There are at least 5 generations of MREs. The initial 12 Meals of the early 80's, 2nd Generation had 24 meals in the late 80's. Post Desert Storm there was another generation. 4th generation was c.2003 and another after c.2010
I may be a bit off on the dates, reflecting when I ate them, vs when they were officially issued.
I served from 1983 through 2014, so I speak from personal knowledge.
Prototype MREs were developed and field tested in the late 1970's. They borrowed some elements from LURP Rations, but mostly were a repackaging of the last generation of C Rations (technically MREs are "C Rations" on the A B C D format; D Rations were the chocolate/protein bars 🍫 deliberately made to not taste good, so the soldiers would not eat them prematurely. Mission Accomplished! Few GIs ate them unless they were starving; usually they threw them away long before they were desperate enough to consider them!).
The initial batches of MREs were mostly just updated C Rations, several were the exact same meals, such as spaghetti and meatballs, or beef ground in spiced sauce. Only 2 meals had a dehydrated main meal, the Beef Patty, and the Pork Patty... BOTH HAD THE CONSISTENCY AND TASTE OF STYROFOAM! about half the meals had dehydrated fruit.
Some MREs had the round crackers from the canned C Rations. Fruitcake was another hold over. Meddlesome twits had petitioned congress to remove All brand names from foods in our rations; cigarettes and prayers too!
Generic everything was about "prison quality" one of the reasons I Love POTUS Ronald Reagan is that he successfully negotiated with congress to allow Brand Name products to be (re)introduced to us. Tobacco, Skittles, and many other things were a Significant Improvement!
During the months of build up during Desert Shield, several hundred thousand US Soldiers and Marines ate the same monotonous meals. Too many cut back severely on eating (and nutrition). Stopgap, interem alternative meals were procured from Dinty Moore and other manufacturers. This led to the 3rd generation MREs in the 90's.
When I was in basic training we mostly had A (or B) Rations, even in the field. We did have some C Rations, and were told about MREs. After OSUT (Basic and AIT) then Airborne School, I was assigned to an XVIII Airborne Corps recon unit; for the most part we had the best, and newest equipment. Sometimes we had experimental stuff too, including Rations.
In Grenada, Honduras, Panama, and elsewhere we mostly had A Rations, or bought food from the locals. On FTXs on Bragg, or elsewhere we had MREs. I supplemented mine with food from REI in Seattle. We also got stuff from SOCOM or the JFK SWC on Bragg. We also Sometimes hunted deer, rabbits and other game, in violation of several regulations.
More later, time to go back to bed.
The hated Ham and Lima Beans was gone, but so too the well liked Tuna Fish (Tuna came back in later versions).