Parshanut for Numbers 31:58

Afterward you shall be gathered to your people. If the war with Midian took place after Moshe’s death, people would say it was not during Moshe’s lifetime because he flattered them, since he was the son-in-law of Yisro and dwelled in Midian for many years. Therefore, Moshe had to take revenge before his death.

They mounted a campaign. In the Sefer HaMitzvos, Ramban counts this as a positive commandment. In the Sifei, the Sages derived that the enemy should not be surrounded on four sides from, “as Adonoy had commanded Moshe.” Ramban held the view that just as it is a commandment to seek peace, so too, it is a commandment to leave them a side to flee so they can save their lives. Rambam, however, does not count this as a positive commandment, because he held the view that this is a strategy of warfare. If Bnei Yisroel were to surround their enemy on all sides, this would cause them to despair that they will fall captive to their enemy, and they will be fortified with all their strength to fight valiantly. Therefore, this was not relevant to be counted as a positive commandment.