This is not an imaginative scenario. Humanity has been through multiple civilizational collapses in its history. The fall of the Roman empire, followed by the J

This is not an imaginative scenario. Humanity has been through multiple civilizational collapses in its history. The fall of the Roman empire, followed by the Justinian Plague and the famines of 536 AD, is one such example. In the wake of the collapse there were thousands tribes that formed, which gradually recoalesced into modern european nation states (and invariably many of those made a pass at superceding Rome's former glory). It would be likely that many groups would inherit heavy weapons but it's not clear that they would be particularly useful. If victory always went to the most heavily armed, then the U.S. would have handily won the Vietnam war without many casualties. And the Afghanistan war while we're at it.