Deuteronomy 20:12

Revised Standard Version (RSV-CI)

But if it makes no peace with you, but makes war against you, then you shall besiege it;

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And if its answer to you is peace and it opens to you, then all the people who are found in it shall do forced labor for you and shall serve you.

and when the Lord your God gives it into your hand you shall put all its males to the sword,