"If it was legal for the US to leave the British Empire when they wished to, why was it illegal for the South to leave the US when THEY wished to? The only difference that I can see is that the South lost the Civil War, but the US won the Revolution (or maybe the BE had too many other wars going to spend the money and time to win it). But if that's the only difference, then law is simply a matter of superior force. In which case, each of us needs as many guns and as much ammunition as he or she can assemble."

There was no United States of America that was ever a part of the British Empire. It was 13 colonies that rebelled and formed the union of states/commonwealths that was then called the USA. And gradually added new territories that later gained statehood.

Texas was part of Mexico that rebelled and with help from fellow anglos in the US became an independent state that later sought admittance into the USA.

Considering that Scotland and Ireland have had more than a few rebellions against "foreign rule", I'm thinking that the BE was not that wonderful for those other than the landed gentry.

Since the USA is an assemblage of states with rights and responsibilities, not to mention legally binding agreements that incorporated them into the union, there is no foreign rule to rebel against. Just a bunch of sore losers who don't seem to be able to play nicely with others.