I seem to remember hearing that Texas has been under five governments: French, Spanish, Independent, US, Confederacy, I think.
I also think states have a right to succeed, considering that "state" is synonymous with "nation," making it clear that this is a union between separate nations.
About the Civil War, keep in mind that the North could have simply let the South succeed, so the war was really about the Union violating Popular Sovereignty. The North didn't want to free the slaves at the start of the war.
@ aum, you'll note that there is actually a state of West Virginia. The Union seemed to be kind of inconsistent about whether the southern states actually succeeded. Officially, they didn't, because they don't have a right to, but at the same time, West Virginia was allowed to separate from Virginia despite not being a state. And what about all the legislation that was passed without representing part of the US?
But really, isn't the Mexican American War more relevant? The one where we started a war for no reason but that we wanted land?