Combat and Conflict in Judaism
Deuteronomy chapter 20 is the starting point for all Jewish discussions about war. It is part of Moses' final speech to the Israelites and is meant to prepare them for the imminent battles with the nations of Canaan (biblical Israel) as well as their future enemies. The chapter includes an assurance of divine support, a list of those exempt from combat, the requirement to offer peace before attacking an enemy, and other rules of war.
The rabbis of the Mishnah (Sotah 8:7; Sanhedrin 1:5) later elaborated on these rules and in doing so distinguished between two types of wars: commanded wars (milhemet mitzvah) and permitted wars (milhemet reshut).
Though there is some ambiguity on the matter, generally speaking, commanded wars refer to wars against the seven nations that originally inhabited Canaan and against Amalek (the nation that attacked the Israelites as they departed Egypt). Permitted wars are expansionary wars undertaken by Jewish kings to secure their boundaries or increase their glory.
According to almost all traditional authorities, the category of commanded wars is no longer relevant. The seven nations of Canaan, as well as Amalek, are considered extinct or unidentifiable. In addition, commentators such as Nahmanides (1194-1270) believed that even the seven nations were only to be fought if they retained their idolatrous practices. This further neutralizes the possibility of invoking a commanded war, as Islam (according to Maimonides [1135-1204]) and Christianity (according to the Me'iri, Rabbi Menahem ben Solomon, [1249-1316]) are not idolatrous religions.