If the Klingons or Romulans defeated and exterminated the other how would the Federation react to such an action?

The Federation is always such a beacon of peace and prosperity and hope so how would they react if the Klingons and Romulans fought another war against other with the Federation being neutral and suddenly one side won decisively and defeated the other and occupied their planets and territories and then started exterminating the losing side so that they could rule peacefully in the future?

Basically destroy the enemy using genocide a few escape and tell the Federation what is happening? The Federation couldn't stop The Dominion when it slaughtered the Cardassians but they won and stopped it, they wouldn't be able to stop it without joining the war themselves, Qapla'