The king of the children of Ammon answered to the messengers of Jephthah, Because Israel took away my land, when he came up out of Egypt, from the Arnon even to the Jabbok, and to the Jordan: now therefore restore those [lands] again peaceably.
Second, he emphasized the fact that Yahweh had given Israel this land. Thus it would have been wrong theologically to allow the Ammonites to take it from them (Judges 11:23-25).