Moab it should seem had heretofore those cities, but now being in the possession of the Amorite, they were to be conquered by Israel by the divine appointment. Genesis 15:18-21. But, Reader, it is sweet to trace all mercies to their fountain head. Hence, therefore, if you consult another scripture on this occasion, you will discover that it was the LORD who gave Israel the victory. See Deuteronomy 2:30-37.
Related Commentaries of Numbers 21:26
Numbers 21:26
26 For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto Arnon.