You keep saying this, but slavery did exist, before there were any States in the Union or even a Union, chattel slavery existed in North America. And significant northerners decided that they simply could not live with countrymen who tolerated slavery without trying to murder them. That tends to upset the prospective murder victims a bit.
Right, and folks on the pro-slavery side sometimes murdered and attacked their anti-slavery contemporaries. William Llloyd Garrison was nearly lynched after giving an anti-slavery speech two decades before Brown became prominent. Minister Elijah Lovejoy was murdered in Illinois around that time. Effigies representing abolitionist leaders were burned and anti-slavery pamphlets were thrown into bonfires. Richard Henry Dana was assaulted by a group of pro-slavery rioters for defending a black man in 1854. Abolitionists often risked their very lives talking about it openly, especially in the slave states.
This wasn't at all unusual. Anti-slavery sentiments were often faced with violence well before the war.
If you're going to point out the extrajudicial violence of the abolitionists, it only seems fair to point our that the pro-slavery side's hands were painted red too.
I would agree but you are still being quite lazy in defining that term. Since both Lincoln and the Republicans platform professed to have neither the power nor the intention of interfering with slavery in the states where it existed (and Lincoln in his inaugural even endorsed a constitutional amendment that would forbid any future amendment to the Constitution to abolish slavery), what aspect of slavery caused this war? Abolishing slavery was not even on the table when the Deep South seceded, although radical northerners indiscriminately murdering southerners, white and black whether they owned slaves or not was on the table. (The first murder victim of John Brown's "army" at Harper's Ferry was Heyward Shepherd, a free black man. How discriminate would the next John Brown army be? And would a Republican President lend the aid of the Federal government to arrest it or sit by and let it engulf the South in a racial holocaust?).
The casus belli of the South could not be more obvious. Nevermind that Lincoln had no such power to outright end slavery, and even if he did, no desire to exercise it. The Lincoln-Douglas debates both threw the election to Lincoln and made him the anti-slavery boogeyman to the South.
James Chestnut led his delegation out of Congress immediately after Lincoln's election, and began organizing the siege of federal installations in the harbor (Chestnut's wife's diary is an excellent read, and he was later a military aide for President Davis). This was before they had even passed a secession ordinance. They weren't even the least bit subtle about it, and slavery was mentioned overwhelmingly in the seceding states' declarations of secession and many other primary sources from the time.
Because the Federal government was violently rebelling against the limitations the Constitution placed on Federal powers. No violation of the Constitution, no secession, at least not in Virginia. When Lincoln called for troops to invade the seceded states, the voters of Virginia wanted nothing to do with that.
In December 1860, a little after Lincoln was elected and more than three months before he was sworn in, South Carolina militia occupied water batteries in the harbor at Charleston and set up other batteries in the attempt to command Fort Sumter. They were outraged that Maj. Anderson had removed Federal property from the arsenal in the city to his position at Fort Moultrie, and when Anderson moved his garrison to Fort Sumter, state authorities complained and began a siege of the fortress. Buchanan refused an indirect summons, just as Anderson refused a direct summons, both in January 1861, two months before Lincoln was sworn in. This was a clear violation of the constitution, as was South Carolina's adherence to a confederacy and as was the act of firing on The Star of the West when they attempted to resupply the fort.
In January, 1861, two months after Lincoln was elected, but two months before he was sworn into office, an armed mob from Pensacola attempted to seize the Federal property at Fort McCrae and Fort Barrancas. Lt. Slemmer drove off the mob and his detachment, which had already removed most of the powder to Fort Pickens, spiked the guns they could not remove and set fire to the rest of the arsenal. The State of Florida sent in militia to seize and hold those forts. The actions of the State of Florida and the State of Alabama in sending troops to Pensacola were clearly a violation of the Constitution.
In February, 1861, in Montgomery, Alabama, seven states formed the Confederate States of America, again violating the Constitution. Article one, Section ten.
All of this occurred before Lincoln was sworn in. The South was making war with the US before Lincoln even had any authority to do anything about it, attempting to seize federal property and firing on federal forces. It was not their property before 1860, and it didn't become their property because their state suddenly claimed it. Was any effort made to reimburse the federal government, and the rest of the states who had paid for those federal installations before their seizure? Did the state congressional delegations make any effort to adjust any such claims before walking out and going home? No. They didn't. These hotheads were just itching for a war.
And really, what was Lincoln to have done? No American president could knuckle under to a pack of terrorists like that. It would be political suicide. You can easily imagine how a modern President would be treated if he caved to this kind of threat. I mean, apply this to modern times. Wyoming has a nuclear missile site. Should they secede, should they be allowed to take those bombs by force and become one of the world's foremost nuclear powers overnight?