At least two million and possibly as much as two and a half million men served in the Union Army; almost all (upwards of 94%) were volunteers. Approximately 20% of these officers, mostly Southerners, resigned and joined the Confederate army.

It is a misconception that the South held an advantage because of the large percentage of professional officers who resigned to join the Confederate army.

At the start of the war, there were 824 graduates of the U. Military Academy on the active list; of these, 296 resigned or were dismissed, and 184 of those became Confederate officers.

However, brigades were changed easily as the situation demanded; the regiment was the main form of permanent grouping. Scott was an elderly veteran of the War of 1812 and the Mexican–American War and could not perform his duties effectively. Although he was popular among the soldiers, Mc Clellan was relieved from his position as general-in-chief because of his overcautious strategy and his contentious relationship with his commander-in-chief, President Lincoln.

Brigades were usually formed once regiments reached the battlefield, according to where the regiment might be deployed, and alongside which other regiments. (He remained commander of the Army of the Potomac through the Peninsula Campaign and the Battle of Antietam.) His replacement, Major General Henry W. He was famous for his victories in the West when he was appointed lieutenant general and general-in-chief of the Union Army in March 1864. Meade) in delivering the final blows to the Confederacy by engaging Confederate forces in many fierce battles in Virginia, the Overland Campaign, conducting a war of attrition that the larger Union Army was able to survive better than its opponent.

Typically, the Department or District commander also had field command of the army of the same name, but some conflicts within the ranks occurred when this was not true, particularly when an army crossed a geographic boundary.

The regular army, the permanent United States Army, was intermixed into various formations of the Union Army, forming a cadre of experienced and skilled troops.

An organization that covered a defined region, including responsibilities for the Federal installations therein and for the field armies within their borders.

Those named for states usually referred to Southern states that had been occupied.

The war proved to be longer and more extensive than anyone North or South had expected, and on July 22, 1861, Congress authorized a volunteer army of 500,000 men.

Lincoln's call forced the border states to choose sides, and four seceded, making the Confederacy eleven states strong.

This group's loyalties were far more sharply divided, with 92 doning Confederate gray and 102 putting on the blue of the Union Army. With the Southern slave states declaring secession from the Union, and with this drastic shortage of men in the army, President Abraham Lincoln called on the states to raise a force of 75,000 men for three months to put down the insurrection.