During the American Civil War, the United States Army, the land force that fought to preserve the collective Union of the states, was often referred to as the Union Army, the Federal Army, or the Northern Army. It proved essential to the restoration and preservation of the United States 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.

Over the course of the war, 2,128,948 men enlisted in the Union army, including 178,895, or about 8.4% being colored troops; 25% of the white men who served were immigrants, and a further 18% were second-generation Americans. 596,670 Union soldiers were killed, wounded or went missing during the war. The initial call-up in 1861 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, the U.S. Army included ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and one of mounted rifles. The regiments were scattered widely. Of the 197 companies in the U.S. 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 U.S. East Coast. There were only 16,367 servicemen in the U.S. Army, including 1,108 commissioned officers. Approximately 20% of these officers, most of them Southerners, resigned, choosing to tie their lives and fortunes to the Confederate army.

Almost 200 United States Military Academy graduates who previously left the U.S. Army, including Ulysses S. Grant, William Tecumseh Sherman, and Braxton Bragg, returned to service at the outbreak of the Civil War. This group's loyalties were far more evenly divided. Clayton R. Newell (2014) states, 92 wore Confederate gray and 102 put on the blue of the United States Army. Hattaway and Jones (1983), John and David Eicher (2001), and Jennifer M. Murray (2012), state that 99 joined the Confederate army and 114 returned to the Union forces.

With the Southern slave states declaring secession from the United States, and with a shortage of soldiers in the army, President Abraham Lincoln called on the states to raise a force of 75,000 troops for three months to put down the Confederate insurrection and defend the national capital in Washington, D.C.

Lincoln's call forced the border states to choose sides, and four seceded, making the Confederacy eleven states strong. It turned out that the war itself proved to be much longer and far more extensive in scope and scale than anyone on either side, Union North or Confederate South, expected or even imagined at the outset on the date of July 22, 1861. That was the day that Congress initially approved and authorized subsidy to allow and support a volunteer army of up to 500,000 troops to the cause.

The call for volunteers initially was easily met by patriotic Northerners, abolitionists, and even immigrants who enlisted for a steady income and meals. Over 10,000 German Americans in New York and Pennsylvania immediately responded to Lincoln's call, along with Northern French Americans, who were also quick to volunteer. As more men were needed, however, the number of volunteers fell and both money bounties and forced conscription had to be turned to. Many Southern Unionists would also fight for the Union army. An estimated 100,000 white soldiers from states within the Confederacy served in Union army units. Between April 1861 and April 1865, at least 2,128,948 men served in the United States Army, of whom the majority were volunteers.