Rashi on Deuteronomy 20:12/רש"י על דברים כ׳:י״ב

1ואם לא תשלים עמך ועשתה עמך מלחמה — Scripture tells you that if it does not make peace with you it will in the end make war against you (attack you) — if you leave it alone and go away (Sifrei Devarim 200:4). (The translation therefore is: AND IF IT WILL MAKE NO PEACE WITH THEE. IT WILL WAR AGAINST THEE).

2וצרת עליה THEN THOU SHALT BESIEGE IT — This implies that you are entitled even to starve it out, to make it suffer thirst and to kill it (the inhabitants) by mortal diseases (Sifrei Devarim 200:5).