Was King George right to fight the insurrection?
The people of Texas would legally and peacefully vote on secession. I wonder... would the federal government be any more successful than King George in stopping the insurrection?
The Constitution defines a form of government, in which the several states agree to operate under an over-arching federal government, as part of a single country. The states have reserved powers, but the states are also subject to the authority of the Federal government. If we assume that oaths mean anything at all, then an oath to uphold the Constitution is an oath to maintain that system of government -- and secession explicitly violates that oath.
This has been a discussion of whether or not the Constitution "allows" secession. It clearly does not, and the Constitution describes the authority by which the federal government may suppress an insurrection.
As a matter of governance under the Constitution, there is no real argument beyond the neo-confederate desire to obscure the real reasons for secession.
We can natter on all day about whether secession is morally justified -- it may or may not be. In the case of the Confederacy, however, there is no real question: the slave-holding roots of secession were utterly immoral.
And make no mistake: it was about slavery. Since you've brought up Texas, note that their referendum was an up or down vote on the Ordinance of Secession. Read it: you'll find that this document was absolutely clear that slavery was the leading cause for their action.