In the waning daylight of the bloodiest single-day battle in American history, a tremendous cheer suddenly resounded from the 23d Ohio Volunteers arrayed across a cornfield in  Sharpsburg, Maryland. The tired men could see the figure of their 19-year-old commissary sergeant driving his mule team through shot and shell to their front lines bearing barrels of hot coffee and food. Every man in that regiment received a cup of hot java—and a second wind—courtesy of young William McKinley, who would later become 25th president of the U.S. Arguments for how the North prevailed during the Civil War are inexhaustible—supplies, manpower, industrial infrastructure. But might we add one more?

Coffee. The winners had plenty. The losers had none.

The Union naval blockade of the Southern coast cut off the supply of coffee to the Confederacy so that between 1861 and 1865, coffee prices soared and a pound of coffee, if it could be found, cost as much as $70 far exceeding a Confederate soldier’s monthly pay of $11. Desperate Johnny Rebs tried roasting dandelion and okra seeds, sweet potatoes and peas, persimmons, and even acorns, trying to come up with a substitute.

Up north, rations were generous at six cups a day, fueling Union soldiers as much as the drink does workers and students today. Union Army camps glowed with thousands of campfires at night, each one with a soldier roasting beans and boiling water. Caffeine-starved rebels sometimes declared an unofficial truce so they could exchange southern tobacco for Yankee coffee. Discarded tin cans with handles of twisted baling wire became personal coffeepots. Soldiers ground the beans with a musket butt and a hard surface, or created a rude mortar and pestle with a tin cup and bayonet. If there was no time to boil water, soldiers chewed on the whole beans as they marched.

Early in the war, Uncle Sam's bean counters figured that soldiers wasted too much time grinding and roasting coffee themselves, the army tried a concentrated instant mixture. Coffee, milk, and sugar were boiled into a thick pudding-like product called “essence of coffee”; reportedly, it looked like axle grease, a “beverage so villainous that the men would not drink it,” wrote Bell Irvin Wiley in The Life of Billy Yank. Later investigations revealed that some contractors used spoiled milk in making the product. Other scoundrels adulterated ground coffee with sand and dirt to increase their per-pound profits.

So the War Department stuck to issuing whole beans, which the soldiers brewed themselves into a beverage “strong enough to float an iron wedge,” as one of Sherman’s veterans described his recipe. “Innocent of lacteal adulteration, it gave strength to the weary and heavy laden, and courage to the despondent and sick at heart.”

But is it possible that coffee's greatest service to the North might have been its early contribution to public health? The discovery that water-borne pathogens caused diseases such as cholera and dysentery lay more than a decade in the future. In boiling their water to brew their java, they were