Many scholars ask, given the depravity they caused, why Hashem did not command Moshe to execute revenge on Moav rather than on Midian. The Yalkut responds that Hashem spared Moav because Moshe Rabbeinu was to be buried within Moav's borders. With the help of this pshat, we can conclude that this idea is expressed in this posuk, seemingly saying: Take vengeance on the Midianites on behalf of Israel. Why not take Moav on? The answer is ..
וַיִּקְבֹּר אֹתוֹ בַגַּיְ בְּאֶרֶץ מוֹאָב מוּל בֵּית פְּעוֹר וְלֹא־יָדַע אִישׁ אֶת־קְבֻרָתוֹ עַד הַיּוֹם הַזֶּה: And He buried him in the valley, in the land of Moav, opposite Beth Pe'or. And no person knows the place of his burial, unto this day. (Devarim 34:6). Thus, refrain from exacting revenge on Moav. This serves as a Hakaras Hatov lesson. Moav saved himself from vengeance by allowing Moshe a place to be buried.