Well sure. The principle they espoused was completely in contradiction to the "Rule by Divine right" basis of the British Monarchy, so there was no question that the Monarchy would attempt to stamp it out.
However, once they established the principle, you would think that subsequent caretakers of the government would then respect it. The principle of Independence was contrary to King George's law, but it was completely in Harmony with that of our own.
Lincoln Rebelled against the Principle as espoused in the Declaration of Independence. The Union were the Rebels.
There was no moral or legal justification for quitting the union.
I got your moral justification right here!
That to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed. That whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness.
As for legal? The Declaration of Independence is the first legal document of the United States of America. The other two, (Articles of Confederation and US Constitution) derive their power and authority from the Declaration, and therefore it is superior to either of them. Indeed, it cites God as it's authority.
US Citizenship began July 4, 1776
That to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed. That whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness.
That circumstance did not exist in 1860. No armies were marching into their towns and seizing their citizens or their property. There was no moral or legal justification for quitting the union. Being angry isn’t justification.