There has been a struggle in this country over state vs. national sovereignty since the beginning.
Under The Articles of Confederation the national government had almost no power. Each state set it's own tariffs, coined it's own money, had it's own military etc. There really was no unification.
The states sent delegates to revise The Articles of Confederation. At the convention the delegates decided that the Articles of Confederation were fatally flawed and drew up the Constitution to replace them.
The goal of the Constitution was to delegate certain powers to the Federal Government and retain the remainders to the states or to the people. There were a block of delegates who believed the Constitution could be construed to grant more power to the Federal Government than the states intended. The Bill of Rights was added to mollify those delegates. Many felt the Bill of Rights was redundant.
At the time leading up to the Civil War, Southern States felt that the Federal Government was encroaching in areas where power had been reserved to the states. We learn a lot about slavery, but it was also about economic and trade policy.
The Southern States believed that since membership in the Union was voluntary they could decide to no longer be members. The seceded and formed the Confederate States of America. They attempted to seize military assets on their territory (Fort Sumpter).
Lincoln settled the argument about Federal vs. State power at the point of a gun. This set the precedent for the Federal Government taking an ever increasing role in the daily lives of the poplulation.
The industrial revolution was making slavery obsolete, it would have died of it's own weight. Lincoln's true legacy was in the change in the balance between state and Federal power.