A war was fought over it. In early 2013, the White House made it explicit in replying to a petition. Yet still there was a sense among at least some Texans that something was amiss. Following U.S. President Obama’s re-election in 2012, citizens of Texas, Louisiana, Alabama, and five other member-states in the U.S. signed petitions for the White House to allow their respective states to secede from the Union. At the time, few people other than the secessionists themselves took the petitions seriously. Yet the underlying contending principles deserve more serious reflection even if no "exit" is anticipated. Most importantly, the matter concerns how and whether the rights of member-states (and majorities of the people, therein) are to be circumscribed in a federal union that leaves said republics semi-sovereign and with residual sovereignty.
The full essay is at "Can an American Member-State Exit?"