Southward it extended towards the frontier of Moab, if indeed the borders of the two nations did not at one time coincide.

When the Israelitish invasion, however, took place, the Amorites under Sihon had thrust themselves between, and had carved for themselves a kingdom out of the northern half of Moab.

The land north of the Arnon became Amorite; but the Ammonite frontier was too well defended to be broken through.
The kingdom of Ammon maintained itself down to the time of David.

At one time, in the days of the Judges, the Ammonites had made the Israelitish tribes on the eastern side of the Jordan tributary to them, and had even crossed the river and raided the highlands of Ephraim.

Under Saul, Ammon and Israel were at constant feud.