Numbers 21:13

And pitched on the other side of Arnon— A river which took its rise from the mountains of Arabia, and fell into the Dead sea; the country of Moab being on the south side of it, and that of the Amorites on the north. It divided itself into several streams, whence is that expression, Numbers 21:14 in the brooks of Arnon. When Moses mentions that Arnon is the border of Moab, it is to shew, that the Israelites had liberty to attack those territories beyond the river Arnon, as not now belonging to the Moabites, upon whom they were forbidden to make war. Deuteronomy 2:9.