The USA was, and at least technically still is, a Republic formed by the joint action of the several SOVEREIGN states.

I believe that Lincoln was wrong when he asserted that secession was unconstitutional. I believe that voluntarily joining a political union does not obviate your ability to dissolve that union (short of war).

However, a reasonable reading of the Constitution suggests that, as with admission, secession would require the concurrence of 2/3 of the states. If you can make a convincing enough case, you should be allowed to depart and set up shop on your own.

Currently, however, secession has been labeled unconstitutional. Once part of the USA, you will not be permitted to depart unless you are able to win your independence through stregth of arms. Well over half a million permanent "votes" were registered to support the perspective adopted by Lincoln, the cause for which they gave the "last full measure of devotion."