When the North American continent was first colonized by Europeans, the land was vast, the work was harsh, and there was a severe shortage of labor. White bond servants, paying their passage across the ocean from Europe through indentured labor, eased but did not solve the problem. Early in the seventeenth century, a Dutch ship loaded with African slaves introduced a solution. Slaves were most economical on large farms where labor-intensive cash crops, such as tobacco, could be grown. By the end of the American Revolution, slavery had proven unprofitable in the North and was dying out. Even in the South the institution was becoming less useful to farmers as tobacco prices fluctuated and began to drop. However, in 1793 Northerner Eli Whitney invented the cotton gin; this device made it possible for textile mills to use the type of cotton most easily grown in the South. Slavery became profitable again.
Torn between the economic benefits of slavery and the moral and constitutional issues it raised, white Southerners grew more and more defensive of the institution. Educated blacks such as escaped-slave Frederick Douglass wrote eloquent and heartfelt attacks on the institution. The outbreak of the Civil War forever changed the future of the American nation. The war began as a struggle to preserve the Union, not a struggle to free the slaves, but many in the North and South felt that the conflict would ultimately decide both issues. Congress passed laws permitting the seizure of slaves from the property of rebellious Southerners. In 1962 President Abraham Lincoln presented the Preliminary Emancipation Proclamation. This document decreed that, all slaves in states would be “thenceforward and forever free.” Furthermore, Lincoln established an institution through which blacks could join the U.S. Army, an unprecedented level of integration at that time. The United States Colored Troops (USCT) served on many battlefields, won numerous Medals of Honor, and ensured eventual Union victory in the war. The thirteenth amendment, abolishing slavery except as punishment for a crime, was passed by the Senate in April 1864, and by the House of Representatives in January 1865.
Reference:
http://www.civilwar.org/education/history/civil-war-overview/slavery.html
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