Until the military reform that followed the Spanish-American War, the regular United States Army was supplemented in time of war by regiments of volunteers, organized by the states but under the direction of the regular army. During the Civil War, 97% of Union forces were in these volunteer regiments.
When President Abraham Lincoln required more troops to fight the Civil War in 1861, he didn`t recruit them directly with federal agents. Instead, Congress passed legislation calling for each state to meet a quota for troops, which it was their separate responsibility to fulfill.
The Confederacy also relied primarily on volunteers. The resulting armies were filled with men who had little idea what kind of combat they were destined for, and were commanded by men with little military training, especially in the North. Conscription was attempted by both sides, but was deeply resented and not very effective.