As the saying goes, an army marches on its stomach, relying on good and plentiful food to fuel its ability to fight.

For contemporary U.S. armed forces in combat, that usually means Meals, Ready-to-Eat, or MREs. U.S. armed forces switched to MREs in the early 1980s, replacing the much-derided canned rations that had sustained troops from WWII through most of the Vietnam war. In September 2018, a specially engineered pizza that can last three years was added to 24 available MRE options, as part of a larger strategy to improve morale (and avoid something called “menu fatigue”).

Throughout history, feeding troops has been a challenge for all of the greatest fighting forces, from the Roman legions to the hordes of Genghis Khan to Napoleon’s chasseurs. Here's how they did it:

Roman armies hunted everything that was available, archaeological remains of wild animals show, says Thomas R. Martin, a professor in Classics at the College of the Holy Cross. From the limited evidence of what the administration in Rome provided the soldiers, he adds, the most important source of calories were carbohydrates: barley or wheat. One source says soldiers were given one pound of meat daily. “For an army, you have to kill 120 sheep a day just for the meat ration. Or 60 hogs,” says Martin.

Whatever the exact amount, it would not be enough to sustain a Roman soldier, who was “a mule more than anything else,” says Martin. They carried very heavy gear, on bad roads, and that’s when they were not expending calories fighting. With their food, they were given wine—a diluted version of what we’re used to—or something closer to vinegar that would help reduce bacteria in their drinking water. For their supply of fat, Roman troops, unsurprisingly, looked to olive oil.

During the Crusades, the average Christian soldier in a siege would have some dried meat and grain to make things like porridge. But this was the food they would have brought with them, supplemented with fruits and vegetables or cheese purchased locally. During the First Crusade, soldiers would have provided their own food stores, which they would have mortgaged their property or sold possessions to buy. Later, during crusades like those in the 14th century, called by Pope Innocent III, deals were made with the Venetian fleet and merchants to keep soldiers supplied.

During battles, "if crusaders got to the Muslim camp they would stop fighting and start eating. And it would cost them the battle. It happened twice at the siege of Acre,” says John Hosler, associate professor of military history at the U.S. Army Command & General Staff College, a medievalist military expert and author of The Siege of Acre, 1189-1191. At one point in the Third Crusade, an observer noted several kitchens in the sultan Saladin’s camp, with up to nine cauldrons each. Those cauldrons were substantial—Hosler points out you could fit four cows’ heads in each. The Christian invaders had nothing comparable.

The Mongol diet “was not gourmet,” says Morris Rossabi, a historian and author of The Mongols and Global History. In the early 13th century, when Genghis Khan was conquering swaths of Asia (mostly in the territory we’d now call China), his horde wasn’t able to carry much. Warriors were supplied by their own households, and as territories were conquered, the Mongols came in contact with foodstuffs like wine. (Their homegrown brand of liquor was fermented mare’s milk called airag, or kumis.)

The Mongolian lands were not particularly arable, nor did the Mongols stay in one place for a long time, so fruits and vegetables weren't staples. The Mongols brought their herds of cows and sheep with them on campaigns. When herds were unavailable, the horsemen would hunt (dogs, marmots and rabbits) or subsist on dried milk curd, cured meat and both fresh and fermented mare’s milk.

At the height of its power, in the late 17th century, the Ottoman Empire was a massive horseshoe around the Mediterranean, including huge swaths of North Africa, the Middle East, modern-day Turkey and Eastern Europe. The Janissaries, elite foot soldiers and bodyguards to the empire's sultan, are considered Europe's first modern standing army.

Janissaries ate well, according to research by Virginia H. Aksan, professor emeritus of McMaster University and a leading scholar of the Ottoman Empire. The soldiers were fueled, she writes, with “fresh baked bread, biscuit when bread was unavailable; a daily meat ration (lamb and mutton) of approximately 200 grams; honey, coffee, rice, bulgur and barley for the horses.”

Above all, the biscuit appears to have held primacy in sustaining the soldiers. One observer noted 105 ovens in Istanbul that were solely dedicated to biscuit-baking for the military. Another wrote angrily about biscuit bakers hoarding excess flour for profit and replacing it with dirt, resulting in the death of many soldiers.

George Washington—along with his quartermaster and commissary general—had major problems feeding the Continental army. Congress lacked taxing authority and thereby lacked the funds to purchase supplies. It was a problem compounded by transportation and other supply issues. The result, according to Joseph Glatthaar, a professor of history at the University of North Carolina-Chapel Hill, was that soldiers would often go days without a ration. “You’d get a little flour and maybe some meat and often the meat is pretty bad,” he says.

In 1775, Congress determined a uniform ration that included one pound of beef (or three-quarters of a pound of pork or one pound of salted fish), and one pound of flour or bread per day; three pounds of peas or beans per week, one pint of milk per day, one pint of rice per week, one quart of spruce beer or cider per day, and a little molasses. (Later vinegar was added.) Because army leaders were rarely able to deliver, soldiers would beg from civilians and supplement with whatever animals they could find. Congress pressured Washington to seize food—paid for with low-value paper currency (effectively an IOU)—but General Washington worried the practice would alienate the colonials.

“On campaign, Napoleon’s soldiers spent most of their time desperately hungry,” says Charles Esdaile, professor of history at University of Liverpool. When all was going to plan, French rations included 24 ounces of bread, a half-pound of meat, an ounce of rice or two ounces of dried beans or peas or lentils, a quart of wine, a gill (roughly a quarter pint) of brandy and a half gill of vinegar. (French measurements are slightly different, so these amounts are approximate.) When bread was unavailable, rough little doughboys would be made from flour, salt and water, baked in the fire, or mixed with stew.

What helped sustain French troops was that European agriculture had switched toward things like the potatoes and corn, which one can eat almost right out of the ground. “French loaves come in long sticks; baguettes,” says Esdaile. “The story is that the baguette was developed so that French soldiers could carry their bread in the legs of their trousers.”

The Union Army in the American Civil War had a standard ration: roughly three-quarters of a pound of meat, a pound of flour or cornmeal, some kind of vegetable and vinegar and molasses. “If you received the standard ration, it would be substantial,” says Glatthaar. “Over time that did not become practical; they began issuing hardtack biscuits called salt cakes, as well as salted meat and dehydrated vegetables.” These were made with flour and water and then dried so they’d last longer.

During campaigns, especially as the Union soldiers moved South, seasonal fruits and vegetables, like apples and sweet potatoes, could be pillaged from orchards and farms. Additionally, soldiers would receive care packages from home, as the Union postal system was fairly reliable throughout the war. As for water, both the Union and Confederate armies relied on lakes and streams.

For U.S. Troops, there were two major types of rations during World War II: the C-Ration (for combat troops) and the K-Ration (less bulky and initially developed for airborne regiments and messengers). “A version of the C-Ration had six containers in one crate, and what’s in a C-ration is going to vary,” says Glatthaar. “You’re going to have a main course—like franks and beans—some cigarettes, some canned fruit, some chewing gum, chocolate bars, some instant coffee, some toilet paper. There’s some processed cheese and some biscuits, but really they’re crackers. And you also get a matchbook.”

Rations, designed to provide three meals—and approximately 3,600 calories—each, were almost universally unpopular. Later, soldiers would get powdered drinks like lemonade and buillion, and eventually sweetened cocoa. K-Rations would have three “meals”: a breakfast, lunch and dinner with four ounces of meat and/or eggs, cheese spread, “biscuits,” candy, gum, salt tablets and a sugary drink. There were also cigarettes, a wooden spoon and toilet paper.

From 1958 to 1981, U.S. rations known as the Meal, Combat, Individual or MCI, were eventually replaced with Meal, Ready-to-Eat (MRE). In Vietnam, these were distributed to combat soldiers in a cardboard box, which contained 1,200 calories through a can of meat (like ham and lima beans, or turkey loaf), a can of “bread” which could be crackers or hardtack or cookies, and a can of dessert, like applesauce, sliced peaches or pound cake.

A full ration could be bulky, so troops often disassembled it, taking what they needed on patrol by placing the cans into socks that they could tie to their packs. In his book, Vietnam: An Epic Tragedy 1945-1975, Max Hastings explains how meals were cooked by punching holes in a ration tin and using a C4 explosive to heat it. Hastings also writes about the pills that troops consumed daily, including a malaria tablet, salt pills that could be sucked on, as well as Lomotil tablets, taken four times per day to control diarrhea brought on by the Halazone troops used to purify their water.