My understanding is that Lincoln fought the war against the South in order to preserve the Union (more so than to free slaves or any other reason), but I don't understand why he (or others) would see the secession of the CSA as any different than the secession of the colonies from Britain a hundred years previous. If Lincoln thought it was fine for the colonists to declare independence in 1776, why wouldn't it be similarly permissible for southern states to do so as well? So, did Lincoln see himself as acting just as Britain had done during the Revolutionary War?
I have seen what sounds like Lincoln's argument that the states could not secede because states entered of their own free will by ratifying the Constitution, but I'm not convinced that means they'd have to remain in the Union a hundred years following, particularly when the Articles of Confederation had only lasted for a short time and the idea of a colony becoming independent was totally new. The Constitution left no legal means for leaving the Union, just as England had left no legal means for leaving the crown.
I have also seen the idea that Britain was infringing on the rights of the Englishmen (Americans) to not be taxed without representation. Couldn't the southern states make similar arguments, that even though they were represented in Congress, they felt that they would never get the rights they deserved because the northern states (and probably later the western ones as well, which wouldn't need slaves) preventing them from it?
As far as economic reasons, it sounds like the South was particularly valuable and possibly in symbiotic cooperation with the North, considering their comparative advantages in different industries. So, I can understand why economically the North would have wanted to retain the South in the Union. That sounds to me like the same explanation Britain would have used, but it doesn't sound like a justification so much as an explanation.
I have read various posts about how the Civil War was actually about Slavery or about States' Rights, so I'm just assuming for this conversation that even if it was "about States' Rights" that the most important right in question was the right to own slaves. Unless I'm misunderstanding, this doesn't seem like an important distinction for my question, since the North's goal was to preserve the Union rather than to do moral good in freeing the slaves. Regardless of why the South wanted to leave, what were the North's arguments justifying their actions when the Colonies had done nearly the same thing to Britain?
So, did Lincoln think he was morally justified in putting down the "rebellion" as he called it, just as King George III had tried to do with the colonies?
So, there's a bit to unpack here. I'll try to break this into chunks, which should all weave together at the end. I'll be drawing heavily from James McPherson, The Battle Cry of Freedom* and Eric Foner, Free Land, Free Labor, Free Men.
Lincoln, the Founding Fathers, and the Idea of America
To Lincoln, the true masterpiece to come out of the Founding Fathers was the Declaration of Independence. While not a legal document or a constitution, it laid out a moral credo for the nation to aspire to. All men are created equal. Lincoln, a Whig-turned-Republican, was more interested in establishing the economic equality of white men than he was a moralistic anti-slavery crusader, but establishing white equality would itself mean outlawing slavery: the common man could not prosper so long as society and the economy were dominated by large landholders that relied on slave labor. Lincoln saw anti-slavery written into the Constitution and the writings (and lives) of the most significant Founders: the Constitution put an eventual end to the slave trade, Washington freed his slaves, Hamilton and Franklin were abolitionists, Jefferson felt crappy about being a slaveholder, and so on. This meant that he was not well-disposed towards Constitutional justifications of secession over slavery, even though he recognized that he could not constitutionally touch slavery where it existed.
Jefferson also took the time to lay a convincing argument that the colonies had been long and systematically mistreated by the Crown and Parliament, justify their invocation of the right of revolution. What could the South do but claim they had lost one election after nearly a century of dominating national politics?
Lincoln and his Constitutional Obligation as President
Lincoln had won a free and fair election in 1860. If anything, he ran at a disadvantage, as Southern politicians conspired to keep his name off the ballot in most Southern states. This meant that his responsibility was to the entire United States, not just the people that voted for him. That included the 500,000-odd Southerners who voted for the Constitutional Union party, those that didn't vote, those that couldn't vote, and yes, those that voted against him. Lincoln also took seriously his oath to defend the Constitution, which included defending from a minority group of secessionists that were attempting to usurp federal authority. While the Constitution did not address secession, the Constitution was not a remaking of the United States itself - it was a change to the existing government of the United States, which came into effect only after a supermajority of the states ratified the document. To get to the true fabric of the United States, one has to look at the union originally established by the several states in the Articles of Confederation, which considered the union perpetual. For practical reasons, this had to be the case: representative government could work when half the country can pick up their ball and go home every time they lose a vote. As Lincoln laid out in the Gettysburg Address, allowing Southern secession would not just prove a mortal blow to the United States, but would doom the very idea of having a government "by the people, for the people." As such, he had no choice as President but to oppose a rebellion that did not appear to speak for the majority of the South's population and had no clear legal basis.
Economics
Of course the South was important to the North, and vice-versa. However, this did not appear to enter Lincoln's calculations all that profoundly; his response would have been the same stopping a poor, isolated state from leaving as it was trying to keep most of the South in the union.
Lincoln's view of himself at the start of the Civil War
Of course, any secession movement anywhere is going to draw parallels to the American Revolution, and the Southern secessionists did so frequently and loudly. However, Lincoln saw himself as the true inheritor of the Founding Fathers' original intentions, with a clear Constitutional and moral duty to defend the government they had built.