Let's be a bit more specific here. No, starting wars of aggression doesn't inherently make one a war criminal. A war criminal specifically someone who conducts war in ways that violate international law. So starting the Iraq War in itself doesn't make Bush and Cheney war criminals, but it's the way it was conducted (and specifically things like the torture program, which flies in the face of the Geneva Convention) that gives them that status.
Per standards established at the Nuremberg Trails, "conspiring to" and "waging the war of aggression", were distinct from "war crimes" and "crimes against humanity" (the two first being "crimes against peace"). But of course those alone are enough o classify them as criminals.
Sadly, we still live in the world where leaders of powerful states are never called into account.
