Deuteronomy 20:10-12

When thou comest nigh unto a city to fight against it, then proclaim peace unto it. And it shall be, if it make thee answer of peace, and open unto thee, then it shall be, that all the people that is found therein shall be tributaries unto thee, and they shall serve thee. And if it will make no peace with thee, but will make war against thee, then thou shalt besiege it