But all those same resolutions said that, in the Declaration of the Union, the internal police of each colony should be recognized as binding. In other words, the origins of American dual federalism are to be found in the resolutions of the revolutionary assemblies authorizing the Continental Congress to declare independence. They declared independence and union together, and there was never any time in which any state acted on the international sphere, having diplomatic relations—and the Constitution itself forbids each state to have any diplomatic action. They could not act independently of the other states in the international arena.

The Real Abraham Lincoln: A Debate (2002)