It is a great irony that anyone in the United States would challenge a state's right to secede from the Union.
The United States of America is the only country in the world that annually celebrates the signing of the Declaration of Independence. Thomas Jefferson's Declaration was an early and influential recognition of the Right of Revolution. And we Americans don't just celebrate it a little -- we celebrate it with fireworks and parades, and we declare this moment in history more in line with our national identity than any other. So it is surprising not only that Americans are unfamiliar with the actual text of the Declaration, but for the most part Americans also do not understand the legal status or historical significance of the Declaration of Independence.
The Declaration is not a law. It was an announcement by an ad hoc organization of like-minded colonists who met first to protest the British Stamp Act ("The Stamp Act Congress"), then again to protest the British Coercive Acts ("First Continental Congress"), and finally to craft a response if Britain ignored their prior protests ("Second Continental Congress").
The legal status of the Declaration, therefore, is about the same as a resignation letter, a loud "Take this Job and Shove It," from colonists who no longer wished to work for the King.
Or perhaps it was more like an employee sit-in: "We are keeping the premises for ourselves, but we are locking out the boss, and claiming the whole thing as our own."
The Declaration was obviously illegal under British Law. There was no legal process by which the colonies might acquire independence. Indeed, the Crown had spent considerable sums setting up the colonies and no doubt expected a good return on that investment, as opposed to having the employees run off with the place like bandits.
So was the Declaration legal or illegal?
The enduring significance of the Declaration is as a statement of Human Rights. The Declaration recognizes two types of rights: First, individual rights (Life, Liberty, Pursuit of Happiness); Second, collective rights (Right of Revolution).
One of the earliest comprehensive statements of Human Rights occurs in the Magna Carta Liberatum ("Great Charter of Liberties") which declares among other things that governments must follow the law, and that individuals are entitled to justice and due process of law.
The Declaration of Independence expanded this significantly by broadly recognizing rights to life, liberty, and the pursuit of Happiness, and by declaring that "Governments derive their just powers from the consent of the governed," which implies the right to dissolve the political bonds and throw off a government to which the people do not consent.
Jefferson's formulation was so compelling that it helped inspire independence revolutions in other countries, including France. The French Constitution of 1793 expressly incorporated the Right of Revolution: "When the government violates the rights of the people, insurrection is for the people, and for every portion thereof, the most sacred of rights and the most indispensable of duties."
Later statements of Human Rights include the Universal Declaration of Human Rights, The Convention Against Forced Labor, The Geneva Conventions Against Torture and War Crimes, Convention on the Rights of the Child, and The Convention on the Elimination of Discrimination Against Women.
Here are the three key takeaways for Americans:
1. The Right of Revolution recognized by the Declaration of Independence is a core element of the American identity. Any legal determination that the laws of the United States do not allow secession is invalid, because it would conflict not only with International Human Rights Law, but also the founding principles of the United States itself.
2. Once a group of people withdraw their consent to be governed, they have the legal and moral right to form their own government. It is a weighty thing to withdraw consent -- there are many benefits that come from federating with other governments, and many costs of going it alone. Some think the UK's Brexit unwise. However, once that consent is withdrawn, the Human Right of self-determination, as recognized in the Declaration of Independence and elsewhere, entitles the people to form their own government.
3. There is no legal or moral justification for any country to use military force against a withdrawing subdivision. They may cajole, coax, and taunt the newly independent government. They may refuse to trade with the new government and force it to rely on other nations for various things. But attack is out of the question. An attack would prove that the underlying "federation" was actually coercive -- a violation of Human Rights, and an unjust government that lacked the consent of the governed.
Some seem to think that the Civil War proves that the United States can or will go to war against a seceding state, reprising the role of King George III and treating the states as economic colonies. My read of the Civil War suggests that the Confederacy attacked the the Union, not vice versa. More to the point, if the Confederate objective were to preserve slavery (itself a Human Rights violation), then it could hardly rely on Human Rights law to defend its behavior.
But in the end, the fact that the United States DID fight a war against the Confederacy does not determine that it was entitled to do so.
And today, 150 years later, if any State or States reasonably determine that the federal government has become oppressive to their freedom, then they are entitled to go their way. If the federal government attempts to prevent them from doing so by force, that would incontrovertibly prove the very oppression that would legally and morally justify Separation and Independence.