I need to arrange a situation in which a major conflict occurs in a relatively advanced civilization and where the fighting is of such a magnitude that all the participants are eventually reduced to the Stone Age or Bronze Age. Is this even possible? And if so how can it be achieved by conventional weapons?

The civilization has no nuclear or biological weapons but is otherwise very advanced (technology can be at any point in the twentieth century within this restriction).

The situation involves an earth like world with an earth like civilization. The details of nations, geography or political boundaries can be adjusted as required, however, the question is aimed at conventional warfare. I imagine the goal would be much easier to achieve with nuclear or biological weapons, but these are out of scope.

Note conventional war not nuclear war so different question from this one:
https://worldbuilding.stackexchange.com/questions/99839/could-humanity-be-blown-back-into-the-stone-age