Originally the Arnon River belonged to the people of Ammon and Moab. But the Amorite king, Sihon, attacked from the north and overran much of their land. Eventually he was stopped at the Arnon River. The Arnon therefore became the boundary between the Amorites to the north and the Moabites to the south (Numbers 21:13).

When the Israelites under Moses were moving north towards Canaan, they conquered the Amorites and seized their territory (Numbers 21:24). Later, when Israel’s territory east of Jordan was divided between its two and a half eastern tribes, the Arnon became the boundary between Israel’s tribe of Reuben and the neighbouring Moabites (Deuteronomy 3:12; Deuteronomy 3:16).