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.
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 this subversive insurrection. 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 men 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 Germans in New York and Pennsylvania immediately responded to Lincoln's call, and the French 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. Nevertheless, between April 1861 and April 1865, at least 2,500,000 men served in the Union Army, of whom the majority were volunteers.
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.S. Military Academy on the active list; of these, 296 resigned or were dismissed, and 184 of those became Confederate officers. Of the approximately 900 West Point graduates who were then civilians, 400 returned to the Union Army and 99 to the Confederate. Therefore, the ratio of Union to Confederate professional officers was 642 to 283. (One of the resigning officers was Robert E. Lee, who had initially been offered the assignment as commander of a field army to suppress the rebellion. Lee disapproved of secession, but refused to bear arms against his native state, Virginia, and resigned to accept the position as commander of Virginian C.S. forces. He eventually became the commander of the Confederate army.) The South did have the advantage of other military colleges, such as The Citadel and Virginia Military Institute, but they produced fewer officers. Though officers were able to resign, enlisted soldiers did not have this right; which meant that they usually had to either desert or wait until their enlistment term was over in order to join the Confederate States Army. While the total number of those is unknown, only 26 enlisted men and non-commissioned officers of the regular army are known to have legally left the army to join the Confederate army when the war began.