[anothergrunt]

Good point and its buttressed by the admission of Texas to the Union. That state (then a separate country) specifically reserved the right to withdraw from the Union as a condition of annexation. This would clearly imply that other states also had the same privilege or the equality of the states of the union would be compromised.

[Ditto]

Here's Ordinance of the Convention of Texas, July 4, 1845 and here is Joint Resolution of the Congress of the United States, December 29,1846 admitting Texas to the Union.

Can you point me to the passage that gives them the right to secede? I can't seem to find it.