Secession happened because 11 states wanted to free themselves from the Union compact. The slavery issue was not the cause but the effect of Federal overreach. The war “happened” because the North couldn’t accept the secession of these 11 states. Lincoln could have prevented war if he had met the the Southern peace delegation in March of 1861. He even admitted to this in his second inaugural address.
The Republican position was further supported by Supreme Court decisions. The first in Cohens vs Virginia 1821 where Chief Justice Marshall wrote;
The people made the Constitution, and the people can unmake it. It is the creature of their will, and lives only by their will. But this supreme and irresistible power to make or to unmake resides only in the whole body of the people, not in any subdivision of them. The attempt of any of the parts to exercise it is usurpation and ought to be repelled by those to whom the people have delegated their power of repelling it.
The actions and statements of President Andrew Jackson further supported the course the Republicans party took. Even Chief Justice Taney, no friend of Abraham Lincoln, believed secession was illegal. He wrote this during the secession crisis;
- SCOTUS Chief Justice Roger Taney, January, 1861
The South contends that a state has a constitutional right to secede from the Union formed with her sister states. In this I submit the South errs. No power or right is constitutional but what can be exercised in a form or mode provided in the constitution for its exercise. Secession is therefore not constitutional, but revolutionary;and is only morally competent, like war, upon failure of justice."
The southern slavocracy appealed to the law of force to achieve their aims and failed miserably.
You are referring to his mention of insurgent agents in D.C. during the time of his first inaugural. Those unrecognized Southern Delegates came to negotiate the breaking up of the Union. What you are saying is that Lincoln meant in his second inaugural that he could have avoided the War if only hed brokered dis-union with the South. You do know that he was first and foremost concerned with preserving the Union? I disagree with your interpretation of Lincolns words.