During the American Civil War, the Union Army referred to the United States Army, the land force that fought to preserve the Union of the collective states. Also known as the Federal Army, it proved essential to the preservation of the United States of America as a working, viable republic.
The Union Army was made up of the permanent regular army of the United States, but further fortified, augmented, and strengthened by the many temporary units of dedicated volunteers as well as including those who were drafted in to service as conscripts. To this end, the Union Army fought and ultimately triumphed over the efforts of the Confederate States Army in the American Civil War.
Over the course of the war, 2,128,948 men enlisted in the Union Army, including 178,895 colored troops; 25% of the white men who served were foreign-born. Of these soldiers, 596,670 were killed, wounded or went missing. The initial call-up was for just three months, after which many of these men chose to reenlist for an additional three years.
When the American Civil War began in April 1861, there were only 16,367 men in the U.S. Army, including 1,108 commissioned officers. Approximately 20% of these officers, most of them Southerners, resigned (willfully abandoning their legal commitments and responsibilities as officers in the military of the United States, to which each and every one of them had sworn an oath), choosing to tie their lives and fortunes to Army of the Confederacy.
In addition, almost 200 West Point graduates who had previously left the Army, including Ulysses S. Grant, William Tecumseh Sherman, and Braxton Bragg, would return to service at the outbreak of the war. This group's loyalties were far more sharply divided, with 92 donning Confederate gray and 102 putting on the blue of the Union Army. The U.S. Army consisted of ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and three 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 Canada–United States border and on the Atlantic coast.