USA

In practice, secession comes for good reason, and the long term optics and/or diplomatic relations of civil war are very bad.

If the public sentiment of Texas truly was that they feel disenfranchised and disconnected from the US, not even a civil war would keep them in line.

This is the real threat in the Basque and Northern Ireland, not the hundreds of lives lost in a one sided military action, but the divides in society, the armed and executive branches, as well as the growing public dissent over the distaste of oppressing your neighbour.