Food stored in a retort pouch have a shorter shelf life if they are stored in a hot location. If stored at 60 degrees Fahrenheit, they will last for about 130 months. But at 80 degrees, they go bad in 76 months. At 100 degrees they are inedible in 22 months, and at 120 degrees or more, will last only a month. Because of this, military MREs are stored in climate controlled warehouses until needed. This way, the MREs can be kept for about ten years, before being used.
Combat rations vary according to food tastes and the military situation. The U.S. expects to send lots of troops overseas on short notice, and to have small units of them operating in a dispersed fashion. This makes American combat troops more effective, and requires MREs that are self-contained. Most other armies don't operate like that, and thus often have field rations boxed for four or more troops. This makes it easier to keep track of and ship the rations. The Israeli field ration comes in a box for four troops. Israeli troops spend most of their time operating close to fast food outlets, so the field rations are more for emergencies than regular eating. Despite the increasing popularity of retort pouch food, a favorite component of field rations in many nations is still the centuries old, and cheap, hard cracker. China uses rice cakes. Countries like Russia, and Middle Eastern nations, use a lot of canned fish and meat. On the other extreme, France delivers quite tasty food in retort pouches (previously this stuff came in large, flat, cans, that could easily be bent and damaged). One thing that has become a lot more common in the past few decades, including the American field rations, is the growing number of accessory items (to the main meal.) Mainly these are snack items, drinks (instant coffee, tea bags, beverage powders) and scented wipes.
Even in armies that use a lot of conscripts, the brass note that quality food is a key morale item. In the past, many armies did not send their troops out of the field for long training exercises because of the expense. But looking at what the United States and Britain have done in Iraq, and elsewhere, and how those troops gained their skills from long periods in the field, more nations are accepting the fact that more training is needed. But it's one thing to scrounge up the cash to pay for lots of field training, but you'd better feed them good while you've got them out there, or you well only have a lot of surly, well trained soldiers. This is not a good thing.
MREs (Meals Ready to Eat), military food delivered in pouches, was first proposed in the 1940s, and the technology was perfected in the 1950s. The retort pouches (thermostabilized, laminated food pouches named after the retort steam cooker) used a three layered pouch that allowed food to be sealed and then cooked inside and never exposed to the air again until eaten. In this way, the food was sterilized in the pouch and the sealed pouch prevented contamination, and loss of flavor, for a long time. NASA began providing retort pouch food for space missions in the 1970s. The U.S. Army picked up on the idea in 1975 and began delivering large quantities of MREs to the troops in 1981. Manufacturing methods were becoming more efficient, thus making MREs cheaper. Campers and hikers became a major market, and new pouch designs eventually made retort pouch food a common product in supermarkets the world over. Since then, most armies have adopted retort pouch food, at least in part, for their field rations. Some canned goods, the favored field ration packaging for over a century, is still used. But because retort pouch technology makes it easier to cook the food in the field, and the MRE style chow tastes better than anything in cans, the retort pouch MRE is taking over.