Numbers 21:26

For Heshbon was the city of Sihon the king of the Amorites,
&c.] His royal city, where he kept his palace, where he had resided for some time, and perhaps some of his predecessors; and therefore being now in his possession when taken by the Israelites, they had a good right and title to keep it, and dwell in it: and indeed this is here given as a reason of it,

who had fought against the former king of Moab;
either the king that reigned before Balak, or some king of Moab, that reigned formerly, against whom one of the name of Sihon, which might be a common name to the kings of the Amorites, as Pharaoh to the Egyptians, had engaged in war:

and taken all his land out of his hand, even unto Arnon;
and had been in the hands of the Amorites some years; and therefore the Moabites had no reason to object to the Israelites dwelling in it, and possessing it, which they had not taken from them, but from the Amorites in a lawful war. And for proof of this, reference is had to the bards and poets of those times, who were the persons that transmitted in verse the history of famous actions to posterity.