No Grab. It can't very well be worries about past war crimes or alleged war crimes that get in the way of America or any other country backing the new Tribunal, since the court won't have any retrospective powers.

In other words, nothing that has happened in the past can ever be brought before the new Tribunal. Only things that haven't yet happened, where the hope is that the existence of the Tribunal might make it a little less likely that they will happen.