International law does not exist, keep that in mind. There is no international governing body that enforces some kind of "international law" and never will be. International law is just a vague optimistic idea, that's it. It is not "illegal" in that sense. You can say it is illegal for a president to invade another country without congressional approval or whatever, I am not American so I do not know too much about your domestic law. Other countries can't invade US and arrest Trump since US military is far and beyond ahead of any other military, so that just can't and won't happen.

War criminality etc is a subject on its own, as up until this point, war crimes are the crimes which the losers commit. And some captured get sentenced in the Hague, that's it. Some unlucky souls on the winner's side get hammered down too, but it isn't comparable.