The city of Sihon: this is added as a reason why Israel took possession of this land, notwithstanding God's prohibition of meddling with them or their land, Deuteronomy 2:9, because it was not now the land of the Moabites, but had been some time since taken from them, and in the possession of the Amorites. The former king of Moab, i.e. the predecessor of Balak, who was the present king.
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.