[wongarsu]

While the world doesn't have a president, you could call the International Criminal Court in The Hague[1] the world's court. If you're a citizen of or on the territory of a member state the ICC has jurisdiction for war crimes (among others), and the majority of countries are member states.

Of course the bigger players all aren't members (US, Russia, China, India). But WWII Germany wasn't part of anything either and we still had the Nuremberg Trials, so it's very much "international law doesn't apply until suddenly it retroactively does".

It's noteworthy though that the ICC's stance on the Superior's Orders defense is not completely clear for the less severe cases.

[1] https://en.wikipedia.org/wiki/International_Criminal_Court