What is the difference between a state seceding from the Union and the colonies seceding from the British Empire?
In my opinion.....nothing.
Thomas Jefferson, America's most prominent secessionist (although he used the word separation instead), wrote the Declaration of Independence as a states' rights document just as much as it was a declaration to separate itself from the British Empire. The Declaration itself states in the first paragraph that:
"these united colonies are...FREE AND INDEPENDENT STATES..."
He believed that each colony was a nation unto itself. The colonies were united for one specific reason. To remove itself from the British Empire. In fact, when the peace treaty was signed to end the War of Independence, King George III signed a peace treaty with each individual state, not with The United States of America.
Furthermore, Jefferson said during his first inauguration that:
"If there be any among us who would wish to dissolve this Union...let him stand undisturbed..."
He also wrote in a letter to Kentucky U.S. Senator John Breckinridge
"God bless them both and keep them in the union if it be for their good, but separate them, if it be better."
In this letter, he was talking about the a secessionist movement that occurred in response to his Louisiana Purchase.
Hey, if it was good enough for Thomas Jefferson, it should be good enough for everyone else, right?