Deuteronomy 20:9-13

And when the officers have finished speaking to the people, they shall appoint commanders of armies at the head of the people.

“When you approach a city to fight against it, you shall offer it terms of peace.

And if it agrees to make peace with you and opens to you, then all the people who are found in it shall become your forced labor and serve you.

However, if it does not make peace with you, but makes war against you, then you shall besiege it.

When the Lord your God gives it into your hand, you shall strike all the men in it with the edge of the sword.