The Signal Corps was created and deployed for the first time, through the leadership of Albert J. Below major units like armies, soldiers were organized mainly into regiments, the main fighting unit with which a soldier would march and be deployed with, commanded by a colonel, lieutenant colonel, or possibly a major. Some units only possessed between four and eight companies and were generally known as battalions.
Regiments were almost always raised within a single state, and were generally referred by number and state, e.g. Regiments were usually grouped into brigades under the command of a brigadier general. Mc Clellan, built and trained the massive Union Army of the Potomac, the primary fighting force in the Eastern Theater.
When the American Civil War began in April 1861, there were only 16,000 men in the U. Army, and of these many Southern officers resigned and joined the Confederate 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.
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.