Jabbok (now Wady Zerka: compare Genesis 32:22) runs eastward under Rabbah of the children of Ammon, thence westward, and reaches the Jordan, 45 miles north of the Arnon. It was between Rabbah and Gerasa that it formed the Ammonite boundary.

And Israel smote him with the edge of the sword,.... Slew him and his army, entirely routed them, and got a complete victory over them; God giving them up into their hands, who otherwise were a very strong, powerful, and warlike people; see Amos 2:9.

and possessed his land from Arnon unto Jabbok; two rivers, the one to the south, the other to the north of his country; the one was the boundary of his country between him and the Moabites, the other the boundary of his country between him and the Ammonites, as it follows:

even unto the children of Ammon; for the border of the children of Ammon was strong; which is given as a reason why the Israelites proceeded no further in their conquest; there was another reason for that, which was the order of the Lord not to distress the Ammonites, nor meddle with them; though Jarchi makes this prohibition to be their strength, Deuteronomy 2:19, but this is given as a reason why Sihon could not extend his conquests further, because it was so well fortified, either by nature or art, or both, by the river Jabbok, by mountains and frontier towns, and particularly by Rabbah, as the Targum of Jonathan suggests, which was their royal city in later times, and a very strong place; see 2 Samuel 12:26.

Israel smote him with the edge of the sword, i.e., without quarter (see Genesis 34:26), and took possession of his land "from Arnon (Mojeb) to the Jabbok, unto the children of Ammon," i.e., to the upper Jabbok, the modern Nahr or Moiet Ammn. The Jabbok, now called Zerka, i.e., the blue, does not take its rise, as Seetzen supposed, on the pilgrim-road by the castle of Zerka; but its source, according to Abulfeda (tab. Syr. p. 91) and Buckingham, is the Nahr Ammn, which flowed down from the ancient capital of the Ammonites, and was called the upper Jabbok, and formed the western border of the Ammonites towards the kingdom of Sihon, and subsequently towards Gad (Deuteronomy 2:37; Deuteronomy 3:16; Joshua 12:2). "For the border of the Ammonites was strong" (firm), i.e., strongly fortified; "for which reason Sihon had only been able to push his conquests to the upper Jabbok, not into the territory of the Ammonites." This explanation of Knobel's is perfectly correct; since the reason why the Israelites did not press forward into the country of the Ammonites, was not the strength of their frontier, but the word of the Lord, "Make not war upon them, for I shall give thee no possession of the land of the children of Ammon" (Deuteronomy 2:19). God had only promised the patriarchs, on behalf of their posterity, that He would give them the land of Canaan, which was bounded towards the east by the Jordan (Numbers 34:2-12; compared with Genesis 10:19 and Genesis 15:19-21); and the Israelites would have received no settlement at all on the eastern side of the Jordan, had not the Canaanitish branch of the Amorites extended itself to that side in the time of Moses, and conquered a large portion of the possessions of the Moabites, and also (according to Joshua 13:25, as compared with Judges 11:13) of the Ammonites, driving back the Moabites as far as the Arnon, and the Ammonites behind the Nahr Ammn. With the defeat of the Amorites, all the land that they had conquered passed into the possession of the Israelites, who took possession of these towns (cf. Deuteronomy 2:34-36). The statement in Numbers 21:25, that Israel settled in all the towns of the Amorites, is somewhat anticipatory of the history itself, as the settlement did not occur till Moses gave the conquered land to the tribes of Reuben and Gad for a possession (Numbers 32). The only places mentioned here are Heshbon and her daughters, i.e., the smaller towns belonging to it (cf. Joshua 13:17), which are enumerated singly in Numbers 32:34-38, and Joshua 13:15-28. In explanation of the expression, "Heshbon and her daughters," it is added in Numbers 21:26, that Heshbon was the city, i.e., the capital of the Amorite king Sihon, who had made war upon the former king of Moab, and taken away all his land as far as the Arnon. Consequently, even down to the time of the predecessor of Balak, the king of the Moabites at that time, the land to the north of the Arnon, and probably even as far as the lower Jabbok, to which point the kingdom of Sihon extended (see Deuteronomy 3:12-13; Joshua 12:5), belonged to the Moabites. And in accordance with this, the country where the Israelites encamped opposite to Jericho, before crossing the Jordan, is reckoned as part of the land of Moab (Deuteronomy 1:5; Deuteronomy 29:1; Deuteronomy 32:49; Deuteronomy 34:5-6), and called Arboth Moab (see Numbers 22:1); whilst the women who seduced the Israelites to join in the idolatrous worship of Baal Peor are called daughters of Moab (Numbers 25:1).

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

(h) The river.

(i) For the people were tall and strong like giants; De 2:20.

21:24 From Arnon - Or, which reached from Arnon; and so here is a description or limitation of Sihon's conquest and kingdom, that is, extended only from Arnon, unto the children of Ammon; and then the following words, for the border of the children of Ammon was strong, come in very fitly, not as a reason why the Israelites did not conquer the Ammonites, for they were absolutely forbidden to meddle with them, Deu 3:8, but as a reason why Sihon could not enlarge his conquests to the Ammonites, as he had done to the Moabites. Jabbok - A river by which the countries of Ammon and Moab were in part bounded and divided. Strong - Either by the advantage of the river, or by their strong holds in their frontiers.

24. from Arnon unto Jabbok-now the Zurka. These rivers formed the southern and northern boundaries of his usurped territory.

for the border of . Ammon was strong-a reason stated for Sihon not being able to push his invasion further.

21:21-35 Sihon went with his forces against Israel, out of his own borders, without provocation, and so ran upon his own ruin. The enemies of God's church often perish by the counsels they think most wisely taken. Og, king of Bashan, instead of being warned by the fate of his neighbours, to make peace with Israel, makes war with them, which proves in like manner his destruction. Wicked men do their utmost to secure themselves and their possessions against the judgments of God; but all in vain, when the day comes on which they must fall. God gave Israel success, while Moses was with them, that he might see the beginning of the glorious work, though he must not live to see it finished. This was, in comparison, but as the day of small things, yet it was an earnest of great things. We must prepare for fresh conflicts and enemies. We must make no peace or truce with the powers of darkness, nor even treat with them; nor should we expect any pause in our contest. But, trusting in God, and obeying his commands, we shall be more than conquerors over every enemy.