Assume a county in State A wanted to join State B. Would that be possible? If so, what would be the procedure? If not, why not?
As said in the US constitution:
Article IV, Section 3, Clause 1 of the US Constitution states: New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
States border changes between two are related to "parts of States", I think. And it is logical - that it should be admitted by both states Legislatures + Congress.
So, seems, that it is possible, but hard to achieve.
Also, there is some fresh example of an attempt.
Yes, but not because of the Statehood Clause. That clause is about adding new states, and is silent about changes to an existing state.
However, if all four of the following groups voted in favor of the change
- The population of the county
- The old state
- The new state
then there is no party left with legal standing in court to sue to stop the change. Any attempt to stop the change in the courts would be thrown out, and the change would be accepted de facto.
There are a few cases where things like this have happened(or come close to happening). You don't hear about the most popular time when it did because is immediately preceded WW2.
Since the mid-19th century, the mountainous region of northern California and parts of southwestern Oregon have been proposed as a separate state. In 1941, some counties in the area ceremonially seceded, one day a week, from their respective states as the State of Jefferson. This movement disappeared after America's entry into World War II, but the notion has been rekindled in recent years.
It doesn't appear that any of these officially seceded, but they came very close. In the articles they talk a little bit about the procedure of how they planned on creating a new state, Worth a look if you were not aware that this had happened. This is a case of counties in states A and B coming together to form a state and not counties moving to a separate state.