Number one, slavery is inherently wrong and cannot be morally defended. You cannot build a legitimate society on slave labor anymore than you can build one based on no property rights. No man, woman or child has a right to rule another human being. The subordination of a parent is nontransferable to the child. Prison sentences cannot be given to a child by the sheer fact that it was brought into the world by an inmate. As such the Southern governments were criminal conspiracies, not legitimate governments. A society that does not acknowledge the equal right to life, liberty and property is one that deserves to be destroyed and remade if it will not be changed peacefully. It is the right of the liberal to impose liberalism on the absolutist (speaking in 18th-19th century political terms) nation because liberalism is the political manifestation of natural law.
The South initiated force against the US Government. As such it was the aggressor. The South did not own Fort Sumpter (sic?) and thus had no right to fire upon a government facility that had neither abridged its natural rights nor initiated force against it. Every US serviceman who died there was murdered, not killed in combat just as much as the Japanese servicemen that attacked Pearl Harbor were murders for killing ~3000 American servicemen when they had not declared a state of hostility with the US.
The Confederate government's member states did not consult their people as to whether they wanted to forfeit their US citizenship. They did not ask them whether they wanted to leave and a state government has no right to leave the Union without a direct affirmation from a majority of its citizens as to whether they wish to make such a fundamental change in their lives. Such an action is akin to the US Government formally nullifying the US Constitution without a vast majority of its constituents affirming the legitimacy of that act.
It would be easy to go on and on about how the US government was actually justified in doing what it did during the Civil War. The Civil War could be easily justified with the size of the US Government today, but not back then. Back in that time it was small and met its constitutional obligations and little more. What the slavers hated was the fact that Lincoln declared openly that the territories had no right to decide on the issue of slavery because they were directly under federal jurisdiction. Imagine back then 5-10 free states being added. Imagine how quickly the Fugitive Slave Act among other things would have been repealed and how quickly the US Supreme Court would have been stocked with anti-slavery minded justices.
All the Confederate supporters say, is states' rights, states' rights. States' rights, what? To "own" another man? To abridge the rights at the state level that are prohibited from being abridged at the federal? Sorry, but there are virtually no powers that can be ethically exercised by one level of government, but not another. Most "states' rights" people are simply tyrants that want to take their war to a new front. America can be better off if we apply the US Bill of Rights to every level of government, not worse off. Ask yourself whether you like local cops violating your privacy any more than you like federal agents do it. Same with gun control laws. Let's stop pontificating about the past and actually get constitutalists in power. The US Constitution is a lot more (Lockean) liberal than any of its proposed alternatives.
1. Slavery is inherently wrong...
No disagreement here. The South did have slavery, which was inherited from the English colonial system. Most of the slaves in America (or their progenitors) entered the country through ports in New England, carried on ships owned by New England Merchants, and were purchased from black African Kings. Yep. It was all the fault of the ignorant bumpkin Southrons.
Ironically, chattel slavery was legitimized in America when a black slaveholder sued to keep from releasing his african servants. He won the case and changed the status of african servants from indentured servants to slaves.
2. The South initiated force against the US Government
After secession, the CSA reached an agreement with Lincoln to allow the foreign US government to maintain possesion of the customs houses in the Southern ports, as long as they were not militarily reinforced. Lincoln was being pressured by representatives of New York and other Northern Port states to put down succession because the tarrif free Southern ports would put them out of business. Several Northern states threatened secession if the "Insurrection" was not put down. Knowing that he could not get away with an outright invasion of the South, Lincoln sent military reinforcements to Fort Sumter, which he had explicitly agreed to NOT do, thus provoking the CSA attack.
3. The Confederate government's member states did not consult their people as to whether they wanted to forfeit their US citizenship
The Southrons were not US citizens. They were citizens of their respective States, which were members of the united States. The legally elected representatives in the State governments did their jobs and voted for that which best helped their citizens. The scalawags were free to move north if they didn't like it, unlike the Southern States, who were compelled to remain in the union by force of arms
PC revisionist history does not change the fact that the primary cause of the war was taxation. At the time of secession tariffs levied on Southern trade accounted for 87% of Federal revenues. Most of the money was spent on "Public Works" in support of Northern industry.
In essence the Federals were developing the North for the benefit of the rich industrialists, and doing so with money taken from the South. If the secession was allowed to stand the Federal government would either have to go bankrupt, cut back on "Public works", or tax those who were benefitting from the "Public works".
Lincoln took a 4th choice and started an illegal war which cost the lives of 600,000 Americans, and launched the Federal Juggernaut that we've come to know and love so much.