In our own day, … as regards the preservation of the security and peace of the whole world … the rulers of individual nations, being all on an equal footing, largely fail in their efforts to achieve this, however much they multiply their meetings and their endeavors to discover more fitting instruments of justice. And this is no reflection on their sincerity and enterprise. It is merely that their authority is not sufficiently influential.
In his encyclical letter Pacem In Terris, I think Pope John was a bit optimistic in his assessment above. Some rulers do not desire security and peace, even for their own people.
But as for those who do, I would agree with Pope John that some more authority than mortals can provide is needed. From the opening sentence of the document:
Peace on Earth—which (humankind) throughout the ages has so longed for and sought after—can never be established, never guaranteed, except by the diligent observance of the divinely established order.