The South had access to a concept that would have ended the war pretty quickly. It's called human rights.
That would have done nothing to end the war. The war was fundamentally because of Lincoln's incorrect view as to the nature of the Union and his unwillingness to accept that the Union was a voluntary confederation of states who voluntarily joined and could voluntarily leave. Instead, he killed over half a million Americans in order to continue to hold to an incorrect view of the nature of the republic, the Nationalist view, which did not even arise until the 1830's. The country was funded under the Compact Theory and would never have been formed if the Nationalist view was the prevailing view.
Even Lincoln wanted to send blacks back to Africa. Ending slavery WOULD have ended the war by bringing England into the war on the side of the Confederacy. The writing was on the wall that slavery was no longer economically viable. Had the southern states been allowed to leave peacefully, slavery would have died out on its own anyway.