We reached that milestone on April 12 with the anniversary of the firing on Fort Sumter. Since then many have focused on states' rights or other complex issues as reasons for the war. But it was about slavery. The states' rights issue rested on the slavery issue.
In his Inaugural Address in 1861, Abraham Lincoln could not have stated it more clearly: "One section of our country believes slavery is right and ought to be extended, while the other believes it is wrong and ought not to be extended. This is the only substantial dispute."