I'm basically done with this discussion, partly because I've lost interest, partly because it's lost focus, partly because my understanding was very incomplete (demonstrated below), partly because well....
But the question was raised about Manuel Noriega, so I thought I'd post the more educated information I gathered, in case anyone else was interested in my finding out more.
I happen to know a man who headed up this competition:
Jessup International Law Moot Court Competition for several years and someone who worked directly under him, so I sent them the following question:
"What enabled the US to try, convict, sentence, and hold Manuel Noriega as a criminal."
I received the following answers...I can inquire for more detail if anyone is interested:
From the underling: "I think, by the way, that the answer is he was convicted of crimes under US domestic law that reached into the US, and that the US didn't recognize him as the leader of Panama, so he didn't have immunity to prosecution."
Response: "That's not really international law though, is it? I mean, it's just domestic law in a red suit and fake white beard."
From the director-dude: "Well it's international law to the extent that international
law (conflicts of law, in this case) permits the US to extend its jurisdiction to acts
which occurred outside of the US but have effects within the US (so-called "objective territoriality.") But yeah, the actual substantive law involved is US criminal law, pure and simple."
More from the director-dude:
"1) In order for a State to try a criminal for a crime, it must have jurisdiction over the person and over the crime.
2) Jurisdiction over the person is typically had by seizing the person. That being said, jurisdictional immunities (like Head of State) may apply. In this case, we argued against the immunity on some basis.
3) Jurisdiction over the crime must come from domestic statute that gives notice to criminals that their acts are subject to US law. In order to be consistent with international law, such jurisdiction must be consistent with one of the recognized bases of national jurisdiction, to wit:
i) territorial (the act occurred in your territory
ii) protective (the act harmed one of your nationals)
iii) nationality (the perpetrator is a citizen of your State)
iv) universal (the crime is one of very very few over which intl law obviates a better link with the State -- eg genocide)
The US also recognizes (though it is controversial) so-called "effects" (or "objective territorial") jurisdiction -- an act which, though occurring outside the US, has effects within the US."
There's some educated and expert detail, if anyone is interested.