Deuteronomy 20:12 in the King James Version
And if it will make no peace with thee, but will make war against thee, then thou shalt besiege it:
Context
11 It shall be, if it gives you answer of peace and opens to you, then it shall be that all the people who are found therein shall become forced laborers to you, and shall serve you.
12 If it will make no peace with you, but will make war against you, then you shall besiege it.
13 When the LORD your God delivers it into your hand, you shall strike every male of it with the edge of the sword;