Quoted in a recent article in the Guardian, Noah Weisbord, an assistant professor at the Florida International University College of Law, who helped draft additions to the statute of the International Criminal Court and was a law clerk to the chief prosecutor of the ICC in the Hague, in an email, said U.S. soldiers could theoretically be tried by the ICC even though the U.S. is not a signatory. But such cases would have to be referred by the U.N. security council and, given that the U.S. has a veto on the council, this makes it very improbable.
Countries that are signatories to the ICC such as Canada or the UK could not arrest U.S. citizens and send them to the Hague.
Weisbord added: “There are, however, a number of fora where U.S. soldiers can be tried for torture. For example, some states have national laws that give their courts universal jurisdiction or other types of robust extraterritorial jurisdiction. This is unrelated to ICC membership. Jurisdiction stems from their domestic laws.”