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).
While not 100% literally accurate I would argue the ICC is an extension of the UN, and the league of allied nations which has always been a rules for thee and not for me type situation. This law and the stance the US took in 2002 isn’t that much of a shift in the practical aspects of their international relations.
There was a shift, domestically, in how the US framed their place in the world though. I’d argue it started before that but it wasn’t so easy to claim benevolence the way military action in WW2 was framed. Imperialism was much more overt and visible to people in the states.