וְאִם־לֹ֤א תַשְׁלִים֙ עִמָּ֔ךְ וְעָשְׂתָ֥ה עִמְּךָ֖ מִלְחָמָ֑ה וְצַרְתָּ֖ עָלֶֽיהָּ׃
Mas se ela não fizer paz com você, mas fizer guerra contra você, então você cercará essa cidade.
Rashi
ואם 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).
Siftei Chakhamim
It will ultimately engage in war against you, etc. And this is what the verse is saying: If it does not settle for peace with you, in the end it will engage in war against you. Therefore you should lay siege to it.
Rashi
וצרת 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).