This question gets brought up from time to time in this sub, and I have answered variations on it a couple of times, as have others. Someone else may want to provide yet another answer. Until then, you may want to refer to this post of mine with a relatively brief summary, along with links to earlier answers that outline the specific issues at play that made the prospect of "peaceful secession" in the long-term, or even in the short-term, unrealistic.

When the original 7 confedrate states seceded from the Union, they took with them all the federal property in their states, including forts and armouries. Southerners assumed they now belonged to the seceding states and many were seized without issue, but two forts in particular became points of contention, Fort Pickens (in Florida) and the better known Fort Sumter (South Carolina). Armed forces tried to seize both from Union garrisons who refused to surrender the forts.

Sumter in particular became a crisis as the small garrison faced thousands of furious South Carolinan beseigers and was rapidly running out of supplies. Lincoln's first decision in office was whether to resupply the fort or surrender it. The former would enrage the forces beseiging Fort Sumter who had already fired on ships approaching the fort and promised to do so again. The latter decision would have enraged the entire North and new Republican party who were desperate to confront the Confederacy rather than appease it. Lincoln made a fairly intelligent decision by informing the state governor that an unarmed ship would resupply the fort with food and water only, if this ship was attacked then it would be considered an act of aggression. The Confederate government realised that they wouldnt obtain the fort peacefully so President Davis ordered an attack on Fort Sumter before the supplies could arrive.

The ensuing battle convinced much of the North that the Confederacy was the aggressor and that the only way to restore the Union was through war. The South meanwhile came to believe that their actions were entirely justified and that they would only obtain recognition of their independence by defeating Union armies. 4 additional slave states seceded after Fort Sumter at the prospect of Union armies invading the South. Both sides became utterly convinced that violence was the only solution.