As I learned it in public schools long before everything became a political football, slavery was not the main focus of Lincoln’s administration initially. It was all about the constitution and the Union. The seceding states, however, had slavery very much on the top of their list of grievances. Lincoln politically embraced abolition as part of his effort to rally the unionists and gain the support of the slaves somewhere in the early stages. I don’t know if it was before or after secession. I suspect it was after the secession because he was focusing on the constitutional issues of dividing the union.

So, yes the civil war was all about slavery. The southern states wanted to expand slavery into the new territories, which was not allowed in the constitution. They wanted to protect and expand slavery as an institution. Some useful information from the Lincoln Home.