Texas is the only state that joined by treaty as an independent county, so it seems reasonable that Texas could maybe repudiate that treaty and leave. I think the example would have been better if they had picked one of the other 49 states, which were created from US territories and were part of the US before they were states. Clearly it is not legal for them to leave; there was even a big war fought to decide that question.
On the other hand apparently the Supreme Court disagrees with me [1], and only 33% of people polled think that Texas can secede [2].
This is not what I have read before about the US constitution. It was very clear at the time the constitution was ratified by the States that they would keep their sovereignty and therefore have a right to secede. Why would it be otherwise, in a country that fought for its independence in the first place?
It's not because there was the Civil War that the Civil War outcome was Lawful. The Southern States had the right to secede and that right was not recognized by Lincoln.
On the other hand apparently the Supreme Court disagrees with me [1], and only 33% of people polled think that Texas can secede [2].
[1] http://blog.constitutioncenter.org/2012/11/constitution-chec...
[2] http://en.wikipedia.org/wiki/Texas_secession_movements