In this very sentence you allude that the war was about leaving, and not about slavery. Bravo. You proved my point that the war was not about slavery.
Again, if the Northern reason for invading was slavery, they would have started in Maryland. Their reason for invading was not slavery, therefore the war was not about slavery.
You’re being disingenuous, as usual. The war began when the eleven southern states seceded and formed the Confederacy. Their reason for doing so was the preservation of slavery. The Southerners said so at the time. Specifically the election of Abraham Lincoln was declared intolerable even before the election. Hostilities began with the firing on Fort Sumter.
The North, in 1861, fought to preserve the Union. By the end of the war the abolition of slavery had been added as an ultimate goal. But the Confederacy started the war. The reason was slavery.