Interestingly, slavery wasn't the biggest issue in question at the start of the Civil War. The main conflict that sparked the war was the Confederate attack on Fort Sumter in 1861. After this attack, the Confederacy seceded from the Union and formed its own nation to engage in battle with the Union.
The underlying issues, however, were much more complex. Slavery was a hot-button issue, particularly in regard to the new territories being added—there was great debate over whether slavery should be allowed in new territories. However, also at the heart of the issue was the complex debate over state's rights. The Confederacy was dedicated to the rights of the states more so than the control of the federal government, and they clung to their state-given right to continue practicing slavery. They believed that, by legislating where slavery was legal and illegal, the federal government was taking power away from the states and was therefore overstepping its boundaries.
As the other answer...
(The entire section contains 3 answers and 680 words.)