European law was cited as it was applicable to purely AMERICAN circumstances. Hammurabian Law and the Torah are also frequently cited in early American court cases.
That is not true-European law and custom was cited as the basis for which title to lands previously owned by natives became subject to the soveriegnty of the government, for example.
It was also the primary grounds for the laws governing commerce on the seas-not an insignificant part of the economy.
Treaties were conducted within the framework of a fairly long history of European international law, not on their own as contracts without any legal framework or requirements. Other countries could (and did) violate international law without violating a treaty, and whether the U.S. was bound by international law was a question considered separate from treaties. This was especially true in regards to disputes with natives.
Taking another read through Amistad....you might want to see how many times they cite foreign laws in the arguments (English especially), and the powers appropriate to "relations between nations". I have not done a complete count, but it looks like
most of the authority cited in this case is foreign, not American.
This case is actually a pretty stereotypical international law case-lots of arguments about which law applies, who gets to decide what the rights involved are, the evidentiary standards to be observed...this reads like a World Court opinion.
Using more foreign laws to explain why the U.S. action here is in accord with international law:
These questions are answered in the negative by all the most approved writers on the laws of nations. 1 Burg. Confl. 741; Story, Confl. 92. By the law of France, the slaves of their colonies, immediately on their arrival in France, become free. In the case of [40 U.S. 518, 554] Forbes v. Cochrane, 2 Barn. & Cres. 463, this question is elaborately discussed and settled by the English court of king's bench.
A big issue here was whether or not the Amistad captives were slaves under Spanish law; since they weren't, the US wasn't required to recognize their status as slaves.
From Story's opinion, the Court ruled on Spanish law, not only US:
It is plain, beyond controversy, if we examine the evidence, that these negroes never were the lawful slaves of Ruiz or Montez, or of any other Spanish subjects. They are natives of Africa, and were kidnapped there, and were unlawfully transported to Cuba, in violation of the laws and treaties of Spain,
Amistad was a ruling on what Foreign laws and treaties required, and based on that ruling, the U.S. wasn't required to act as Spain demanded.
Edit: You can read the Amistad opinion here:
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=CASE&court=US&vol=40&page=518Again, to make clear-Amistad did not reject English and Spanish law. They affirmed it-the ruling was that because these slaves would not be property under Spanish law (due to an international treaty on abolishing the slave trade), they therefore could not be considered property under the treaty between the U.S. and Spain.
The Court is quite explicit in pointing out that if the Spanish law had been different, these folks would've been returned to Spanish jurisdiction, even though the U.S. law presumes them to be free.
