DEUT988 Offer peace to cities upon which you are about to lay siege. Mercy is a very commendable character trait, and it is fitting for our nation to exercise it even towards our enemies. Even those who worship idols should be allowed to live, as long as they abandon their deities. A further benefit of the mitzvah is that after these people surrender, they become our servants and pay taxes to our king. If we were to kill these people, even when they are willing to pay taxes and be our servants, not only would it be a senseless waste, it would be patently cruel.