International Law as Part of Our Law

What did the United States Supreme Court mean when it famously said, “International law is part of our law, and must be ascertained and administered by the courts of justice of appropriate jurisdiction, as often as questions of right depending upon it are duly presented for their determination”? Perhaps the Court was suggesting that, in an interdependent world, United States courts should not decide cases without paying “a decent respect to the opinions of mankind,” in the memorable words of the Declaration of Independence. The framers and early Justices understood that the global legitimacy of a fledgling nation crucially depended upon the compatibility of its domestic law with the rules of the international system within which it sought acceptance. Their recognition seems both prudent and sensible. Even today, for any nation consciously to ignore global standards not only would ensure constant frictions with the rest of the world, but also would diminish that nation's ability to invoke those international rules that served its own national purposes.