Oh yes, it was nothing if not complicated. Bear with me while I go into the complexities to back up my assertion, which I can only justify by supplying the missing qualifications.

SECESSION

As far as the Civil War is concerned, Fort Sumpter was the explosion, but it was lit by more than one fuse, each of them going back a long way. It’s not just difficult to say who started the war, it’s even a little tricky to pinpoint WHEN it started.

Union soldiers refused to evacuate because South Carolina had declared a secession Lincoln considered illegal; he saw it as an insurrection justifying military occupation. As far as South Carolina was concerned, once-fellow-American soldiers had now become foreign invaders of a sovereign power. Hence the bombardment. South Carolina believed the North had started a war. For the North it was the other way around: A rebellious state militia had attacked Federal troops. The more radical Republicans believed this wasn’t just rebellion; it was outright treason; the harshest punitive measures were called for, including executions, once the war was won.

This raises an interesting question. IS secession treason, according to the Constitution? This article argues that it isn’t.

https://www.abbevilleinstitute.org/blog/was-secession-treason/

The Constitution doesn’t mention “secession.” Three states explicitly demanded a right to secede as a condition for ratifying it--including Virginia, but also New York. Wasn’t the War of Independence an act of secession from the British? The Constitution tried to resolve the conflict between states’ rights and the central government—the “States” in “the United States of America” vs. the “United.” Clearly it didn’t exactly succeed. Each new territory polarized Southern and Northern states into Slave and Free. Kansas became a battleground. Pierce fanned the flames through pro-Southern partisanship. Legislative compromises repeatedly failed and matters got out of all legal and political control. The momentum towards war became unstoppable. The powder keg was already stuffed with powder. It was just a question of who would strike the match.

As for the “right” to secession: the Constitution does enjoin the President, as his first duty, to protect and defend the Constitution of the United States; the Uniformed Services oath adds “from its enemies, foreign and domestic”—and Trump is Commander-in-Chief. This duty “Trumps” every other consideration, including that of states’ rights. (The Constitution IS the United States in this sense: It CONSTITUTES it.) The Constitution doesn’t have to mention secession, because by seceding a state makes itself a domestic enemy. The verbal promises of a right to secession, made to three states in the heat of Constitutional Convention negotiations, isn’t actually included in any section, paragraph or clause IN the Constitution. Secession isn’t mentioned, but neither is any RIGHT to secession.

ABOLITION

Lincoln had been arguing against slavery at least as far back as the Cooper Union speech (in a scholarly way, with respect to the legalities of the Slave vs. Free State question). But early in his Presidency he was relatively quiet on the issue (partly to avoid alienating the Free Soil Democrats he had to work both with and against). His emphasis was on preserving the Union. But the South seceded primarily because the Republicans were clamoring for abolition, and they didn't distinguish between the Radicals and the more moderate President. This was an “existential threat’ to the South’s feudal economy and its ruling class. Secession was its pre-emptive threat-response. The point of no return had already been passed; no Free-Soiler compromise was acceptable to either side. Sooner or later Lincoln had to make abolishing slavery not just an explicit cause for fighting the war, but a Cause. After Antietam that’s exactly what he did with the Emancipation Proclamation.

Constitutional authority goes only so far in extreme situations. There is also the MORAL issue, above and beyond even the highest Law of the Land. Slavery in modern terms is a crime against humanity (or human rights violation). John Brown (a kind of AntiFa extremist lunatic, it’s true) believed the South was already waging war against the human race and God just by enslaving blacks. If the South had seceded in order to preserve a Baal cult, would we say they had a state’s-rights justification for doing so, on Constitutional grounds—i.e., the Tenth Amendment? Would having a child-sacrifice cult be among the rights not enumerated in the preceding Amendments and therefore reserved to the states?

When Lincoln proclaimed “a new birth of freedom” he was perhaps implying that the Civil War was a second Revolution. (This was Gore Vidal’s take in "Lincoln".) The Emancipation Proclamation was the Declaration of Independence (All men are created equal) re-written to explicitly include black people.

PRESERVING THE UNION

So did the North or the South start the war? The South thought they were defending themselves against foreign invasion: “Northern aggression.” Lincoln believed it was the South that had started HOSTILITIES. He refused even to recognize that a Secession had happened. He treated it as an uprising by states (or even just the leaders of those states) who were nevertheless still part of the Union, whether they liked it or not. The uprising had to be met with force; in that sense Lincoln’s war was a form of riot control.

It’s my personal opinion that Lincoln was right to consider the Secession an insurrection (so, for the matter, is this Election’s voter fraud); and on the ground there’s not much difference between staging an insurrection and starting a war. He was also right to fear that if the South managed to secede, it would start a balkanizing chain-reaction ending in complete disintegration of the nation the Patriots fought for and our Forefathers founded. A house divided against itself cannot stand.