Rabbi Gordon - Shoftim: 7th Portion

Hashem also said to give the option of peace. "When you approach a town to attack it, you shall offer it terms of peace" (Deuteronomy 20:10).

The sages debated whether that sentence is in reference to the wars against the seven nations, or only other wars. Rashi sides with the position that it is in reference to the wars against the seven nations (mandatory wars), saying that the call to peace must even be given to them. The command to wipe out the seven nations only applies if they reject the terms of peace.

Rashi's position is accepted as law. The Rambam (Laws of Kings and Wars 6:1): "War is not conducted against anyone in the world until they are first offered peace (and refuse it), whether this is a Discretionary War or a War of Mitzvoh, as it says, “when you come close to the city to fight with it, you shall call to it to make peace” (Deut. 20:10)." Reply