The American Civil War was primarily fought over differences of opinion about the right of the federal government to determine whether slavery was legal in individual states; the hostile action that started the actual war was the Confederate attack on Fort Sumter in South Carolina, which occurred on April 12, 1861. The secession of seven Southern states after the inauguration of Abraham Lincoln as president triggered the war.
Lincoln had pledged to limit the legal institution of slavery to the existing states that allowed it. By the time of the Civil War, most of the Northern states had already outlawed slavery while most of the Southern states protected it. The major debate was whether the practice would be allowed to expand into the new western territories, which were beginning to become states. Lincoln planned to forbid that, but the Southern states that would become the Confederacy thought it should be up to each individual territory. Many Northerners also supported complete abolition, believing that the federal government should outlaw slavery in all the country's states and territories.
While slavery was the major contentious issue, there were also more complex issues at play. The two regions were at odds economically. The Northern region was primarily industrial while the South was mostly agricultural. There were disagreements about trade agreements, tariffs and other economic issues.