I'm going on the record here: the worst thing to happen to this country, bar none, was the war that ran 1861–65. The income tax? A ballooning federal debt? Fiat money? Overreaching executives? Election fraud? Eminent domain? Suppression of free speech? The decline of federalism? All bad things, but not as bad as what they have in common.
In the first place, there's obviously the human toll: 600,000+ dead, when the combined population of North and South was only around 30 million. It's still the deadliest war in American history. It also brought us atrocities like Sherman's march and horrors like Andersonville.
The long-term results, however, have also been devastating. Good ol' Abe Lincoln <choke> gave us the country's first income tax, a huge federal debt, lots of fiat money, and an excellent model for future presidential power grabbers by the names of Roosevelt, Wilson, Roosevelt, and Johnson. He rigged elections in Maryland, carved out a chunk of Virginia and made it a state, and denied habeas corpus to Northern journalists and activists imprisoned for speaking against his policies. And in the end, he destroyed what Lord Acton called "the most efficacious restraint on democracy that has been devised," that is, Federalism—the power of the States to check the central government.
Why was the war fought? Before you mechanically respond with what they indoctrinated taught you in public school, consider what Lincoln said in his first inaugural address, just months before hostility broke out:
I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.
[Slavery] has been a mighty instrument not for evil only, but for good in the providential order of the world.
Hmmm... that sure doesn't fit the narrative. But what about the Emancipation Proclamation?
All persons held as slaves within any State, or designated part of a State, the people whereof shall then be in rebellion against the United States shall be then, thenceforward, and forever free.
If Lincoln really cared about slavery, might he have actually freed the slaves in the states that formed his government? He didn't. The Emancipation Proclamation freed only the slaves in states that Lincoln's government did not control, while slavery continued in Northern states like New Jersey and Maryland. For Lincoln, the liberation of slaves was pure pragmatism, a means of winning the war.
Besides, if slavery was the issue, shouldn't the U.S. have followed the example of virtually every other country in the Western Hemisphere? Of all the countries west of the Atlantic, only Haiti and the United States experienced widespread violence associated with abolition, and in Haiti it was slave revolts. Brazil—which had approximately as many slaves as the United States—let slavery go with hardly any strife at all. How? By degree: as slaves escaped to free states, the rest of the country gave up. Result? Free slaves, without slaughter of men or self-government.
Such a path in the United States would have been more difficult due to the Fugitive Slave Law (a burden Brazil didn't have to deal with). But those who truly wanted the end of slavery were, decades prior, advocating secession—of the North. "No Union with Slaveholders" was their cry. If they had succeeded, they would have abolished slavery and the Fugitive Slave Law in the North, clearing the road for a gradual and humane abolition of the peculiar institution throughout the continent.
They were opposed, however, by a faction in the North, known as Republicans, that wanted something else: control over the South in the form of perpetual union. They had blinked during the crisis of 1832, and they weren't going to repeat the mistake. One member of this faction, Lincoln, articulated their position this way:
No State, upon its own mere motion, can lawfully get out of the Union.
Lincoln's support of "perpetual union" and rejection of self-determination and self-government (except, conveniently enough, in the case of West Virginia!) was so strong that he was prepared to send hundreds of thousands of young men and civilians to their deaths. He had apparently forgotten about some rebels who, 85 years prior, had defied a powerful nation and seceded from it, claiming that their rights were under attack from a foreign power. They had made declarations like:
When a long Train of Abuses and Usurpations, pursuing invariably the same Object, evinces a Design to reduce them under absolute Despotism, it is their Right, it is their Duty, to throw off such Government, and to provide new Guards for their future Security.
The States were, of course, shocked, because Lincoln's belligerence on the issue was completely opposite to the assurances they had received when asked to sign the Constitution in the first place. Virginia had even said in its ratification of the Constitution:
The powers granted under the Constitution, being derived from the people of the United States, may be resumed by them, whensoever the same shall be perverted to their injury or oppression.
It is absolutely ridiculous to think that most of the states would have accepted the Constitution if they had thought that it meant "perpetual union." Talk of the right of secession throughout early American history was widespread, even though never acted on until 1860. For just one example, in 1815, a group of New Englanders opposed to the War of 1812 formed a convention in Hartford and discussed the merits of secession. Even Daniel Webster, later a fierce advocate for perpetual union at the expense of the South, spoke supportively of the convention in a speech to Congress.
Sadly, the tyrants won the day in America's Second War of Secession, and proceeded to subjugate the vanquished. By abolishing slavery in the worst way possible—instantly, by force, and without regard for the individual interests of owners and slaves, they created other problems. Would racism to the point of violent resistance (by the KKK and others) have been an issue in the South had it been abolished peacefully and legislatively? Would racism still be a weapon wielded to make political points?
We can't know for sure, but what is sure is that nearly all of our modern federal government's egregious violations of the Constitution have precedent during Lincoln's term in office. The closest thing to truly free government ever seen on earth was poisoned on his watch. It staggered on for a few more decades, until Teddy Roosevelt dug the hole, Woodrow Wilson put the wood box together, and Franklin Roosevelt nailed it shut.
(if you want more on this, do yourself a favor and read Lord Acton's treatment on the subject and, if you're slightly more ambitious, Jeffrey Hummel Roger's Emancipating Slaves, Enslaving Free Men: A History of the American Civil War)