Our Founders' respect for the Law of Nations does not mean that it must somehow supersede the Constitution.
Nor does the Law of Nations authorize secession "at pleasure."
See my post #218 for more...
§ 160. Nullity of treaties which are pernicious to the state.
Though a simple injury, or some disadvantage in a treaty, be not sufficient to invalidate it, the case is not the same with those inconveniences that would lead to the ruin of the nation. Since, in the formation of every treaty, the contracting parties must be vested with sufficient powers for the purpose, a treaty pernicious to the state is null, and not at all obligatory, as no conductor of a nation has the power to enter into engagements to do such things as are capable of destroying the state, for whose safety the government is intrusted to him.
Law of Nature and Nations, Book II, Chap XII
The law of nature, “which, being coeval with mankind and dictated by God Himself, is, of course, superior in obligation to any other. It is binding over all the globe, in all countries, and at all times. No human laws are of any validity, if contrary to this.”
Alexander Hamilton, The Farmer Refuted 23 Feb. 1775 http://www.heritage.org/initiatives/first-principles/primary-sources/alexander-hamilton-the-farmer-refuted