We are not a signatory to the Rome Statute, the treaty (and never have been)that defines “crimes against humanity” and establishes the International Criminal Court, though we(the US) apparently had a hand in making it.
If the US were to join the ICC next year, could someone from the United States be prosecuted in that court for things done prior to the US signing the treaty?
Not unless the US specifically provides for that option in acceding to the Statute of Rome which establishes the Court.
Article 11 provides:
The US signed the Rome Statute on 31 December 2000, but on 6 May 2002 notified the Secretary General that it did not intend to become a party and would not be proceeding to ratification. SFAIK it remains a signatory (so that, if it did decide to ratify, it would not need to sign again.)
As Northern Piper says, only if the US stipulated that when ratifying the Treaty.
(A person from the US could, of course, be prosecuted even now for a crime within the jurisdiction of the Court that is committed in a state that is party to the Statute. The fact that the US hasn’t ratified the Statute means the Court can’t deal with crimes committed within the US; it doesn’t mean that US citizens enjoy a worldwide immunity.)
I was actually reading that (the Statute of Rome) on the UN website because I was curious as to what “crimes against humanity” actually is. Just focused on that and didn’t think to look at the rest 
“Crimes against humanity” is not a concept that was invented in the Rome Statute, or for the purposes of the ICC. It goes back to the nineteenth century, and first turns in a legal instrument in the Hague Conventions on the Laws of War (the first of which was signed in 1889), which cited the “laws of humanity” as one of the constraints that should determine what conduct is permissible on the part of a state conducting a war.
“Crimes against humanity” was one of the charges laid at the Nuremburg and Tokyo tribunals after the Second World War. The Nuremburg Charter defined the concept to include “murder, extermination, enslavement, deportation, and other inhumane acts committed against any civilian population, before or during the war, or persecutions on political, racial or religious grounds in execution of or in connection with any crime within the jurisdiction of the Tribunal, whether or not in violation of the domestic law of the country where perpetrated”.
Subsequent attempts at definition obviously haven’t been specific to the Second World War or the the jurisdiction of the Nuremburg Tribunal, but the basic elements have been very similar.
Thanks for that.
I will admit that I haven’t explored the topic deeply. Like Northern Piper said, thanks for the lesson.