Adam Clarke on Joshua 13:25

Half the land on the children of Ammon - This probably was land which had been taken from the Ammonites by Sihon, king of the Amorites, and which the Israelites possessed by right of conquest. For although the Israelites were forbidden to take the land of the Ammonites, Deu 2:37, yet this part, as having been united to the territories of Sihon, they might possess when they defeated that king and subdued his kingdom.