Thank you, sw, for the citation to The Paquete Habana, 175 U.S. 677 (1900). This is one of a number of cases in which SCOTUS had to deal with the extraterritorial reach of the Constitution and international law in the context of US global expansion and the Spanish-American War. Those "Insular Cases" have a good deal of relevance to present-day "Small Wars" issues.

From the Paquete Habana syllabus:

Under the Act of Congress of March 3, 1891, c. 517, this Court has jurisdiction of appeals from all final sentences and decrees in prize causes, without regard to the amount in dispute and without any certificate of the district judge as to the importance of the particular case.

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. For this purpose, where there is no treaty and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations, and, as evidence of these, to the works of jurists and commentators, not for the speculations of their authors concerning what the law ought to be, but for trustworthy evidence of what the law really is.
which makes a number of points:

1. The Federal courts were given jurisdiction in prize cases by statute (which was within their constitutional mandate).

2. The international law issues must be "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." This ties into point 1.

3. "Customary international law" comes into play only "where there is no treaty and no controlling executive or legislative act or judicial decision." In short, it is a last resort in arriving at a rule of decision in a case where the court has jurisdiction.

4. In making that determination, the court looks, "as evidence of these, to the works of jurists and commentators, not for the speculations of their authors concerning what the law ought to be, but for trustworthy evidence of what the law really is." Modern punditry is filled with "what the law ought to be".

The Paquete Court embarked on a multipage historical quest (starting in 1403) to determine what "customary international law" was concerning fishing vessels seized in prize cases. Nonetheless, three members of the Court differed and concluded that "customary international law" in this area was neither "customary" nor "authoritative":

In is needless to review the speculations and repetitions of the writers on international law. Ortolan, De Boeck, and others admit that the custom relied on as consecrating the immunity is not so general as to create an absolute international rule; Heffter, Calvo, and others are to the contrary. Their lucubrations may be persuasive, but not authoritative.

In my judgment, the rule is that exemption from the rigors of war is in the control of the Executive. He is bound by no immutable rule on the subject. It is for him to apply, or to modify, or to deny altogether such immunity as may have been usually extended.
[Page 175 U. S. 721]
Exemptions may be designated in advance or granted according to circumstances, but carrying on war involves the infliction of the hardships of war, at least to the extent that the seizure or destruction of enemy's property on sea need not be specifically authorized in order to be accomplished.
The lesson to be learned is that incorporation of "customary international law" is best done by definitive joint executive and congressional action. In that, I join Judges Hogan and Brown.

Regards

Mike