Many factors contributed to the beginning of the Civil War, most notably the divisive issue of whether or not slavery would be allowed in the territories of the United States. At the time, the United States was rapidly expanding westward, and the newly-elected President Lincoln vowed that the newly-acquired land would not allow slavery.
CivilRights.org notes that there were several issues central to the Civil War, in addition to slavery. Free and slave-states held different positions regarding State's rights and taxation. The North and South had different economic interests that led to conflict, the North focusing on industry and manufacturing and the South driven by agriculture.
Several southern states seceded from the Union and formed the Confederate States of America, or the Confederacy, in reaction to Lincoln's anti-slavery decree. Although the succession was not formally recognized, the Union had to react. However, when the Confederate army captured Fort Sumter in 1861, and rose its own Confederate flag, the Civil War began.
South Carolina, Georgia, Florida, Mississippi, Alabama,Louisiana, Texas, Virginia, Arkansas, North Carolina, and Tennessee all seceded over the course of the war. The Union captured Confederate President Jefferson Davis in Georgia on May 10, 1865, ending the war and beginning the long end of slavery, as an institution in the United States.