Numbers 21:24

And Israel smote him with the edge of the sword, and possessed his land from Arnon unto Jabbok, even unto the children of Ammon: for the border of the children of Ammon was strong.

Israel ... possessed his land from Arnon unto Jabbok, even unto the children of Ammon. The brook х naachal (H5157)] Jabbok (called "the river of Gad," 2 Samuel 24:5), now Nahr ez-Zerka, which rises near the ruins of Rabbath-Ammon (Amman), east of the mountains, flows by a deep chasm between the towering range of Jebel 'Ajlun and the Belka, and enters the Jordan opposite Nabulus. The territory comprised within these two rivers was in early times called the land of Gilead" (Numbers 32:1; Deuteronomy 34:1), now the Belka-which is divided into two nearly equal halves by the Wady Hesban, the northern portion extending from it to the Jabbok, while the southern portion between Wady Hesban and the Amen is again intersected by the Wady Zerka Main (Meon), which empties itself into the Dead Sea (Robinson's Physical Geography,' p. 79).

The whole country, embracing all the low-lying as well as highland regions east of the Jordan, had been formerly occupies by the descendants of Lot, until the portion known as the province of Belka, belonging to Moab, was, century before that time, invaded and seized by the chief of a wandering tribe of mountaineers (the Amorites) from southern Judea, where they were intermingled with the Anakim, and who, taking advantage of the decay of the Moabite nation, effected a permanent settlement between the territories of the two brethren, Moab on the south and Ammon on the north. The two rivers, the Arnon and the Jabbok, formed the boundaries of his usurped possession.

For the border ... of Ammon was strong - a reason stated for Sihon not being able to push his invasion further. The Ammonite territory cannot be exactly defined, because that people were a tribe of Bedouin marauders; but the country north of the Jabbok, as far as mount Hermon (Jebel esh-Shiekh) was called anciently "the land of Bashan," now the Hauran. The Ammonites were driven further to the east; because another invasion of Amorites was made on the land north of the Jabbok, which resulted in the establishment of a second Amorite kingdom in Bashan.