Obviously there is the moral issue with slavery, but it seems hypocritical to me that the North did not allow the South to secede from the United States when the United States was the result of an secession in the previous century.
I am not sympathetic to the South, but I am curious if Lincoln or the Union had an ethical and moral reasoning against state self-determination. In Canada, each province is allowed to declare independence if a majority wants it, but as far as I know the US states cannot.
President Abraham Lincoln argued that the constitution of the United States was a binding contract that prohibited any state from unilaterally seceding from the federal republic of which it was a member.
An important difference between the 13 American colonies declaring their independence from the British Empire in 1776 and the Confederate states declaring their independence from the United States of America in 1861 is that the former involved colonies declaring their independence from an imperial government while the latter involved a group of states declaring their independence from a federal republican government.
The 13 colonies did not have any representation in the British parliament, and as a result, they did not have a say in the policies that the British put in place in the American colonies. When the British imposed taxes or placed limits on the western expansion of colonial settlement, the colonials had no democratic representatives to argue on behalf of their interests in the British parliament, nor could they expect to be given that right when the American declaration of independence was being drafted.
The southern states had been part of a federal republic in which they did have democratic representation in their country's legislative chambers. The southern states had represented a powerful political force as a region since the founding of the U.S.A., but the general election of 1860 showed that a political party with almost no support outside of the northern and western states could still win the Presidency and a majority of the legislative seats in both houses of Congress. This leads into the topic of what caused the southern states to secede and form the Confederacy, which I don't feel qualified to discuss in detail.
Follow up questions: Did the United States government officially address the justifications for secession written in the Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union? If so, did they dispute that the claims were true or that the claims justified secession or both?
How does this compare to any official response the British gave to the justifications laid out in the Declaration of Independence?
An interesting side topic to this, is how the secession of counties that made up West Virginia after the Civil War was recognized as a new state. So you have it well established that states can't secede from the federal union, but counties can secede from their own state.
Given that the American Revolution was basically a secession from the British Empire
But it wasn't. The confederates tried to secede. They attempted to leave the union legally, pretending that they had every right to do so under the Constitution (they didn't).
The colonies declared independence. They rejected the rule of the crown entirely. They absolutely knew that under British law this was treasonous and would be met with force.