‘ And the king of the children of Ammon replied to the messengers of Jephthah, “Because Israel took away my land when they came up out of Egypt, from Arnon even to Jabbok, and to Jordan. Now therefore restore those lands again peaceably.” '
The reply came back just as haughtily. The king demanded the return to him of lands now under the control of Israel, (the territory of Reuben and Gad), which he claimed had once belonged to Ammon, (although Israel had taken them from the occupying Amorites, not from Ammon). But that land had never belonged to Ammon, it had belonged to Moab (Numbers 21:26). Thus it is clear that the king of Ammon was here linking Moab with himself in his claims. In other words he was speaking on behalf of an Ammonite/Moabite confederacy. (Compare Deuteronomy 2:9; Deuteronomy 2:19 where both were to be treated as the same by Israel because they were the descendants of Lot. They were ‘brothers'). Furthermore he knew perfectly well how impossible it would be for Jephthah to acknowledge his claims. It would be to admit that Reuben and Gad should pay tribute to him in perpetuity. That would be worth sacrificing a bit of Gilead for, especially as he could always come back for that later and no doubt would levy tribute, but he did not really expect it to happen. What he hoped was that Jephthah would give up and pay tribute.
“From Arnon even to Jabbok, and to Jordan.” The river Arnon was the border between Moab and the Reubenites (the latter living where the Sihon and the Amorites were previously - Numbers 21:13), and the river Jabbok was the northern border of Gad (formerly of Sihon and the Amorites - Numbers 21:24). It was true that the land occupied by Sihon and the Amorites had formerly belonged to Moab (Numbers 21:26-30) and was captured by the Amorites from Moab, and then by Israel from the Amorites and populated by Reuben and Gad. But it had not been Moab's for a long time and all saw it as having belonged to the Amorites by right of conquest.
One special importance for us of this statement is that it demonstrates that this attack was therefore not only by Ammon, but included Moab who regularly allied themselves with Ammon, for they were ‘the descendants of Lot' and therefore ‘brothers'. (Compare Judges 3:13 where Moab was predominant and mentioned alone all the way through except in Judges 11:13. Had it not been for Judges 11:13 we would have thought it was Moab alone. It was a general tendency among kings of a confederacy to take credit to their own people. Also see Deuteronomy 23:3).
We do not of course have here the full text of the message from the king of Ammon, and what follows suggests strongly that he did indeed stress that the territory had belonged to Moab their ‘brother' and that it was theirs because it belonged to Chemosh their god.