Judges 11:13

Because Israel. The King of Ammon stated the cause of his quarrel very distinctly. He claimed that all the land of Gilead between the Amon and the Jabbok really belonged to the Ammonites, and demanded its surrender as the only condition of peace. This was not in accordance with the facts. The Israelites had been forbidden to war against the Moabites and Ammonites (Deut. 2:9, 19), so they had passed around Moab; and also avoiding the territory of Ammon, which lay along the desert, they had crossed the Amon into the territory of Sihon, king of the Amorites. Sihon, it is true, had wrested this territory earlier from Moab and Ammon (Num. 21:21-30; Joshua 13:25), but that was a question with which the Israelites had nothing to do. When they captured the land it belonged to someone else.

From Amon even unto Jabbok. The deep, rocky ravine of the Amon formed the southern boundary of Israel, separating the tribe of Reuben from Moab. It is about 50 mi. from the Amon north to the Jabbok River. The hill country and moorland of this region had been given to the tribes of Reuben and Gad because it was suited for grazing. This strip of territory between the Jordan and the desert was about 20 mi. wide.