States started seceding before he was even inaugurated.
Yep. By the time he was sworn in as the 16th President of the United States - South Carolina, Mississippi, Georgia, Florida, Alabama, Louisiana, and Texas had all left the Union, formed their own government, and elected Jefferson Davis as their president. Virginia, Arkansas, North Carolina, and Tennessee would join them just over a month after Inauguration Day.
The movement toward Civil War began before Abraham Lincoln was even born. That journey was initiated at the Constitutional Convention in 1787 when important questions (namely the one concerning slavery) were left untended to or glossed over entirely.