Judges 11:13 - Joseph Benson’s Commentary on the Old and New Testaments

Because Israel took away my land The land was not theirs when the Israelites conquered it, but the land of Sihon, king of the Amorites. For as to the country of the Ammonites, God expressly charged the children of Israel not to meddle with it, Deuteronomy 2:19. It is true, this land, which they now claimed, had formerly belonged to the Moabites, but Sihon had made a conquest of it, and driven them out, as we read Numbers 21:26.