As the USA minister of defence, he is in all likelihood responsible for crimes against humanity/war crimes, for instance, the killing of people on small boats in the Caribbean, and the hundreds of school girls in Iran, to name just two cases. But who could sue him; it seems it is usually countries that do this, but could eg an organization of international lawyers do this? What would it take for an individual to get him sued? (Asking for a friend).
If the location where those crimes happened would be in a country that signed up to the international criminal court (treaty of Rome), it could actually work, eg Venezuela or Caribbean countries. It would in any case greatly hamper Petey’s freedom of travel across the world, as he might risk arrest.