The military campaign against Midian serves as a vital spiritual closure rather than a mere tactical battle. Following the Midianite attempt to corrupt the Israelites from within using Balaam's advice, the time has arrived to break their power and uproot the impurity they planted [רש ר הירש, בעלי ברית אברם].
God's command to initiate this war is firm and decisive, yet it carries a deep undercurrent of pain. This hardship is twofold. On one side, Moses faces the sadness of his own approaching death, which is directly tied to the conclusion of this very battle. On the other side, God Himself experiences sorrow, as it were, over having to decree the end of His faithful servant's life [אור החיים]. Because of this heavy personal cost, God's communication carries a hidden layer of comfort and appeasement, gently encouraging Moses to accept his final earthly mission [אור החיים].
The instruction also clarifies that Moses must not fight this battle alone. Because the call to avenge is framed as a personal task, one might mistakenly assume that Moses, known as a mighty warrior, is expected to wage the war by himself. Instead, he is specifically guided to gather and involve the entire nation [אור החיים]. However, when Moses relays the message to the Israelites, he shares only the call to arms. He deliberately hides the fact that his own death will immediately follow their victory. He does this to protect the people's morale, ensuring they do not refuse to fight out of sadness for their leader [אלשיך].
Leading this specific battle offers a profound personal rectification for Moses. During the earlier sin at Baal Peor, Moses displayed a moment of hesitation and did not immediately act with zeal for God regarding the public sin of Zimri and Cozbi, which forced Phinehas to step in. God now grants Moses the opportunity to correct this by acting with personal zeal against the Midianites right before he passes away [אלשיך].
The exclusive focus on punishing Midian rather than Moab stems from their differing motives. While the Moabite women seduced the Israelites out of simple lust, the Midianite women executed a calculated plot designed specifically to trap the nation in idol worship. Because the Midianite scheme caused a much deeper and more severe fracture in the covenant with God [אברבנאל], their defeat becomes a necessary condition to fully wipe away the sin and clear the Israelites of their guilt [בעלי ברית אברם].