Moses was commanded to wage war against the Midyanites before his death, because they were complicit in Bilam’s plot to seduce the Israelites to engage in forbidden sexual activity and to worship idols. Although Moses could have delayed carrying out the mission, thereby postponing his death, he did not delay; he immediately commanded the Israelites to go out to battle.

Rabbi Yehuda says: Had Moses sought to live several more years, he could have lived. As the Holy One, Blessed be He, told him: “Avenge the vengeance of the children of Israel on the Midyanites; then you shall be gathered to your people” (Numbers 31:2). The Holy One, Blessed be He, made Moses’ death contingent on taking vengeance against Midyan. The verse serves to inform you of the praise of Moses, who did not say: So that I will live longer, I will delay the taking of vengeance by the Israelites against the Midyanites. Rather: Immediately, “Moses spoke to the people, saying: Select from among you men for the army, and they shall be against Midyan” (31:3). “One thousand per tribe from the thousands of Israel were provided, twelve thousand mobilized soldiers” (31:5). The term “were provided” [written in the passive] indicates that it was against their will. Since the verse made Moses’ death contingent upon vengeance against Midyan, the Israelites said: Will we go to attack Midyan and have Moses die? They refrained from going. The Holy One, Blessed be He, said to Moses: Draw lots for the tribes to determine who should go to battle, and the soldiers will be provided against their will. (Tanĥuma, Matot)