And Israel smote him with the edge of the sword. This was the first time that generation had seen war, if we except the uncertain episode of the king of Arad, and they could have had no weapons but such as their fathers had brought out of Egypt.
It was, therefore, a critical moment in their history when they met the forces of Sihon, confident from their recent victory over Moab. We may suppose that Joshua was their military leader now, as before and after.
From Arnon unto Jabbok. The Jabbok, which formed the boundary of Sihon on the north towards the kingdom of Og, and on the east towards the Ammonites, is the modern Zerka: it runs in a large curve northeast, north-west, and west, until it fails into Jordan, forty-five miles north of the mouth of the Arnon.
Even unto the children of Ammon: for the border of the children of Ammon was strong. This is perhaps intended to explain rather why the Amorites had not extended their conquests any further, than why the Israelites made no attempt to cross the border of Ammon; they had another and more sufficient reason (see Deuteronomy 2:19).
Rabbah of Ammon, which stood upon the right (here the eastern) bank of the Upper Jabbok, was an extremely strong place which effectually protected the country behind it, even until the reign of David (see on 2 Samuel 11:1-27, 2 Samuel 12:1-31).