If genocide is an actual possibility of the future, then no people on earth—least of all, of course, the Jewish people, in Israel or elsewhere—can feel reasonably sure of its continued existence without the help and the protection of international law.
Numerous events since Arendt wrote those words (such as the international community’s passiveness in the face of the genocide in Rwanda, for example) have undermined the idea of international law. Far from being an unreachable blessing, an international tribunal governed by international law would be a catastrophe that humanity has thus far managed to avoid.