This is one of the most complex, contentious, and debated questions in American history. The shortest answer is that the secession of the lower Southern states, combined with Lincoln's response to it, caused the war. The diplomatic maneuvering that followed secession culminated at Fort Sumter, when Confederate batteries launched a bombardment of Federal forces. This, along with Lincoln's call for troops to put down the rebellion (which led to the secession of the rest of the Confederate states) started the war. But the causes of secession, which led to the war, go far deeper than that. They revolve around the cental isssue of slavery, especially debates over the expansion of slavery into the west. Slavery was at the heart of a divergence in interests between the North and the Upper and Deep South that exploded with the crisis of 1860-61.
The threat of civil war in the United States had been brewing for decades. A feud simmered in Congress as the Southern states slowly lost power since the votes were often determined over issues such as states' rights and slavery. As free states grew to outnumber slave states, Southern politicians routinely brought up the possibility of secession. The Presidential election of 1860 proved to be the last straw for Southerners: Their candidate, former Vice President John Breckenridge of Kentucky (a pro-slavery candidate from a slave state) was defeated by Republican Abraham Linncoln of Illinois (a free state). Lincoln did not advocate an immediate end to slavery, but he believed it was an antiquated system whose existence had few years remaining. He pledged to see that no further slave states would be added to the Union, and the South believed that Lincoln's term would crush the political power long held in the South. The South worried that cotton, their leading economic product, would suffer; the South had yet to industrialize their cities as Northern cities had. Shortly after Lincoln's election, South Carolina became the first Southern state to secede from the Union. Other Southern states followed, and they soon banded together as the Confederate States of America. When Confederate troops assaulted the Union-held Fort Sumter in Charleston Harbor, the war was on. Lincoln, determined to reunite the Union, soon sent troops into the Southern states, and all-out war continued for four long years.