[mac_truck]

Under what circumstances, if any, is any state able to opt out of its membership in the Union?

With the consent of Congress..the same way you came in.

[NTHockey]

Actually, the original 13 states ratified(i.e, approved) the Constitution in state legislatures. They created a federal government. They needed no approval from Congress SINCE CONGRESS NEVER APPROVED THEIR STATEHOOD.

Thus, VA, NC, SC, GA broke no law when they seceeded. Likewise, TX was free to seceed having been unique among the states as a republic prior to staehood.

[CodeToad]

“With the consent of Congress..the same way you came in.
“

I don’t remember reading about that in the Constitution. Can you point us to that specific clause?