What is the idea that states choose to enter the union and can choose to leave it?

Answer 1

The competing theories for secession were the "choice theory" and the "just cause" theory, and both were examined after the Nullification Crisis of 1832, when South Carolina sought to avoid the collection of federal tariffs in the state. The general principle of secession is part of the collective term "states' rights."

No state was forced to enter the union of The United States. At the time that the Constitution was being decided upon, it was decided that 9 of the original 13 colonies would have to accept it before it would take effect. But initially, only the colonies that adopted it were bound by it. Eleven of the original colonies accepted it (North Carolina and Rhode Island joined later) and so the original United States consisted of 11 states. The United States of which Washington was first elected president consisted of these 11 states only.

However, there is no mechanism for a state to leave the union. This was a difficulty faced by Lincoln at the beginning of the Civil War. The Confederate States wanted to secede, but Lincoln concluded that they could not, under the law as established by the Constitution.

Voters could, if they wished to, amend the Constitution so that there is a mechanism for a state to follow in order to leave the union.