A lot of bickering back and forth over whether Lincoln was a devil or a saint. I have a different twist on this debate: American contract law states that whenever people form a contract, it must be on a voluntary basis. There can be no duress (force) or the contract is null and void.
If states enter into a contract to become part of the USA , and are not free to leave that association under threat of harm, then such contract is null and void. Such an agreement would give disproportionate power to one party at the expense of the other party.