הלכה אויף דברים 20:20

We [begin] sieges against idolater cities [at least] three days before the Shabbat. But we make war against them each and every day, and even on Shabbat, until we conquer them – even if it is an optional war. They learned from the oral tradition (on Deut. 20:20), “until it be subjugated” – and even on Shabbat. And there is no need to say [this] regarding a commanded war. And Yehoshua only conquered Jericho on Shabbat.

We may besiege a non-Jewish city (even) on Shabbos⁴⁴Other versions read: “but may not begin within three days before Shabbos” so as not to violate Shabbos. and we may make war with them even on Shabbos, as it says, “until it falls” (Deut. 20:20). We do battle on Shabbos whether it is a War of Mitzvoh or a Discretionary War.