Everywhere.
"Whoever considers, in a combined and comprehensive view, the general texture of the constitution, will be satisfied that the people of the United States intended to form themselves into a nation for national purposes. They instituted, for such purposes, a national government complete in all its parts, with powers legislative, executive and judiciary, ad in all those powers extending over the whole nation."
Supreme Court Justice James Wilson, 1793
Where does the Constitution --allow-- for secession?
The Congress has the clear power to provide for the common defense and general welfare. If secession is inimical to those interests, the neccesary and proper clause empowers the Congress to act.
At least that's what Jefferson Davis thought.
Walt