The nation tried for years to negotiate its differences. Consider the great compromises regarding admitting new states during the years prior. The intractable problem was politico-economic: slave states had a political advantage through the slave populations which enabled them to gain economic advantage. If one had to pick one cause of the war, it would be the fight to prevent the expansion of slavery, not slavery per se.
Yes it is true negotiation went for years and it would seem negotiation was a dead end. That said, l wonder if the southern states thought the north would actually mount an invasion? In the end the southern slave owners lost everything. If they thought that was a strong possibility maybe negotiation would have worked.
Slavery in the U.S. was going to have to end one way or the other. Lincoln realized this, but had no real Constitutional way to do it. Even after the Civil War had begun, there was no legal way for him to touch slavery in areas not actually in rebellion against the Federal government. Read the text of the Emancipation Proclamation.
How to end slavery peacefully and Constitutionally is hardly ever considered. Great Britain actually paid slave owners in their colonies to end slavery there. How long it would have taken for this to be politically feasible, no one knows. Taxpayers who don’t own slaves paying higher taxes to free the slaves...? Few opposed slavery that much.
Negotiation could well have prevented the war, but as for ending slavery, I don’t know. But it appears that many blacks believe the world and history revolves around them and them alone.