World War I was the first time in American history that the United States sent soldiers abroad to defend foreign soil. On April 6, 1917, when the United States declared war against Germany, the nation had a standing army of 127,500 officers and soldiers. By the end of the war, four million men had served in the United States Army, with an additional 800,000 in other military service branches.

Once war was declared, the army attempted to mobilize the troops very quickly. The fatigued British and French troops, who had been fighting since August 1914, sorely needed the relief offered by the American forces. In May 1917, General John Joseph "Black Jack" Pershing was designated the supreme commander of the American army in France, and the American Expeditionary Forces (AEF) were created. Pershing and his staff soon realized how ill-prepared the United States was to transport large numbers of soldiers and necessary equipment to the front, where supplies, rations, equipment, and trained soldiers were all in short supply. Since even the transport ships needed to bring American troops to Europe were scarce, the army pressed into service cruise ships, seized German ships, and borrowed Allied ships to transport American soldiers from New York, New Jersey, and Virginia. The mobilization effort taxed the limits of the American military and required new organizational strategies and command structures to transport great numbers of troops and supplies quickly and efficiently.

Although the first American troops arrived in Europe in June 1917, the AEF did not fully participate at the front until October, when the First Division, one of the best-trained divisions of the AEF, entered the trenches at Nancy, France. Pershing wanted an American force that could operate independently of the other Allies, but his vision could not be realized until adequately trained troops with sufficient supplies reached Europe. Training schools in America sent their best men to the front, and Pershing also established facilities in France to train new arrivals for combat.

Throughout 1917 and into 1918, American divisions were usually employed to augment French and British units in defending their lines and in staging attacks on German positions. Beginning in May 1918, with the first United States victory at Cantigny, AEF commanders increasingly assumed sole control of American forces in combat. By July 1918, French forces often were assigned to support AEF operations. During the Battle of St. Mihiel, beginning September 12, 1918, Pershing commanded the American First Army, comprising seven divisions and more than 500,000 men, in the largest offensive operation ever undertaken by United States armed forces. This successful offensive was followed by the Battle of Argonne, lasting from September 27 to October 6, 1918, during which Pershing commanded more than one million American and French soldiers. In these two military operations, Allied forces recovered more than two hundred square miles of French territory from the German army.

By the time Germany signed the Armistice on November 11, 1918, the American Expeditionary Forces had evolved into a modern, combat-tested army recognized as one of the best in the world. The United States had sustained an estimated 360,000 casualties in the First World War, including 126,000 killed in action and 234,000 wounded. In less than two years the United States had established new motorized and combat forces, equipped them with all types of ordnance including machine guns and tanks, and created an entirely new support organization capable of moving supplies thousands of miles in a timely manner. World War I provided the United States with valuable strategic lessons and an officer corps that would become the nucleus for mobilizing and commanding sixteen million American military personnel in World War II.