The Union Army was the land force that fought for the Union during the American Civil War. It was also known as the Federal Army, the U.S. Army, the Northern Army and the National Army. It consisted of the small United States Army (the regular army), augmented by massive numbers of units supplied by the Northern states, composed of volunteers as well as conscripts. The Union Army fought and eventually defeated the smaller Confederate States Army during the war which lasted from 1861 to 1865. About 360,000 died from all causes; some 280,000 were wounded.

When the American Civil War began in April 1861, there were only 16,000 men in the U.S. Army, and of these many Southern officers resigned and joined the Confederate States Army. The U.S. Army consisted of ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and one of mounted infantry. The regiments were scattered widely. Of the 197 companies in the army, 179 occupied 79 isolated posts in the West, and the remaining 18 manned garrisons east of the Mississippi River, mostly along the Canadian border and on the Atlantic coast.