The Arnon, now the Wady Mojeb, an impetuous torrent, divided the territory which remained to the Moabites from that which the Amorites had wrested from them, Numbers 21:26.

Arnon - Another river which takes its rise in the mountains of Moab, and, after having separated the ancient territories of the Moabites and Ammonites, falls into the Dead Sea, near the mouth of Jordan.

From thence they removed, and pitched on the other side of Arnon,.... A river on the borders of Moab:

which is in the wilderness that cometh out of the coasts of the Amorites; according to Jarchi, they went round the land of Moab, all to the south and east, and came not into the border of Moab, as Jephthah said, Judges 11:18 but before they came hither they had a station at Almondiblathaim, Numbers 33:46.

for Arnon is the border of Moab, between Moab and the Amorites; a river which divided these two countries, and bounded them; and Moses is the more particular in this account, to show that the Israelites took nothing from the Moabites, but what the Amorites had taken from them, they being charged not to distress the Moabites and Ammonites, Deuteronomy 2:9, see Jephthah's defence, Judges 11:15.

The next encampment was "beyond (i.e., by the side of) the Arnon, which is in the desert, and that cometh out of the territory of the Amorites." The Arnon, i.e., the present Wady Mojeb, is formed by the union of the Seyl (i.e., brook or river) Sade, which comes from the south-east, not far from Katrane, on the pilgrim road, and the Lejum from the north-east, which receives the small rivers el Makhreys and Balua, the latter flowing from the pilgrim station Kalaat Balua, and then continues its course to the Dead Sea, through a deep and narrow valley, shut in by very steep and lofty cliffs, and covered with blocks of stone, that have been brought down from the loftier ground (Burckhardt, pp. 633ff.), so that there are only a few places where it is passable; and consequently a wandering people like the Israelites could not have crossed the Mojeb itself to force an entrance into the territory of the hostile Amorites.

(Note: It is utterly inconceivable that a whole people, travelling with all their possessions as well as with their flocks, should have been exposed without necessity to the dangers and enormous difficulties that would attend the crossing of so dreadfully wild and so deep a valley, and that merely for the purpose of forcing an entrance into an enemy's country. - Ritter, Erdk. xv. p. 1207.)

For the Arnon formed the boundary between Moab and the country of the Amorites. The spot where Israel encamped on the Arnon must be sought for in the upper part of its course, where it is still flowing "in the desert;" not at Wady Zade, however, although Burckhardt calls this the main source of the Mojeb, but at the Balua, which flows into the Lejum. In all probability these streams, of which the Lejum came from the north, already bore the name of Arnon; as we may gather from the expression, "that cometh out of the coasts of the Amorites." The place of Israel's encampment, "beyond the Arnon in the desert," is to be sought for, therefore, in the neighbourhood of Kalaat Balua, and on the south side of the Arnon (Balua). This is evident enough from Deuteronomy 2:24, Deuteronomy 2:26., where the Israelites are represented as entering the territory of the Amoritish king Sihon, when they crossed the Arnon, having first of all sent a deputation, with a peaceable request for permission to pass through his land (cf. Numbers 21:21.). Although this took place, according to Deuteronomy 2:26, "out of the wilderness of Kedemoth," an Amoritish town, it by no means follows that the Israelites had already crossed the Arnon and entered the territory of the Amorites, but only that they were standing on the border of it, and in the desert which took its name from Kedemoth, and ran up to this, the most easterly town, as the name seems to imply, of the country of the Amorites. After the conquest of the country, Kedemoth was allotted to the Reubenites (Joshua 13:18), and made into a Levitical city (Joshua 21:37; 1 Chronicles 6:64).

The Israelites now received instructions from the Lord, to cross the river Arnon, and make war upon the Amoritish king Sihon of Heshbon, and take possession of his land, with the assurance that the Lord had given Sihon into the hand of Israel, and would fill all nations before them with fear and trembling (Deuteronomy 2:24-25). This summons, with its attendant promises, not only filled the Israelites with courage and strength to enter upon the conflict with the mightiest of all the tribes of the Canaanites, but inspired poets in the midst of them to commemorate in odes the wars of Jehovah, and His victories over His foes. A few verses are given here out of one of these odes (Deuteronomy 2:14.), not for the purpose of verifying the geographical statement, that the Arnon touches the border of Moabitis, or that the Israelites had only arrived at the border of the Moabite and Amorite territory, but as an evidence that there, on the borders of Moab, the Israelites had been inspired through the divine promises with the firm assurance that they should be able to conquer the land of the Amorites which lay before them.

From thence they removed, and pitched on the other side of Arnon, which is in the wilderness that cometh out of the coasts of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites.

21:13 On the other side - Or rather, on this side of Arnon, for so it now was to the Israelites, who had not yet passed over it. Between Moab and the Amorites - Though formerly it and the land beyond it belonged to Moab, yet afterwards it had been taken from them by Sihon. This is added to reconcile two seemingly contrary commands of God, the one that of not meddling with the land of the Moabites, Deu 2:9, the other that of going over Arnon and taking possession of the land beyond it, Deu 2:24, because, saith he, it is not now the land of the Moabites, but of the Amorites.

13. pitched on the other side of Arnon-now El-Mojib, a deep, broad, and rapid stream, dividing the dominions of the Moabites and Amorites.

21:10-20 We have here the removes of the children of Israel, till they came to the plains of Moab, from whence they passed over Jordan into Canaan. The end of their pilgrimage was near. They set forward. It were well if we did thus; and the nearer we come to heaven, were so much the more active and abundant in the work of the Lord. The wonderful success God granted to his people, is here spoken of, and, among the rest, their actions on the river Arnon, at Vaheb in Suphah, and other places on that river. In every stage of our lives, nay, in every step, we should notice what God has wrought for us; what he did at such a time, and what in such a place, ought to be distinctly remembered. God blessed his people with a supply of water. When we come to heaven, we shall remove to the well of life, the fountain of living waters. They received it with joy and thankfulness, which made the mercy doubly sweet. With joy must we draw water out of the wells of salvation, Isa 12:3. As the brazen serpent was a figure of Christ, who is lifted up for our cure, so is this well a figure of the Spirit, who is poured forth for our comfort, and from whom flow to us rivers of living waters, Joh 7:38,39. Does this well spring up in our souls? If so, we should take the comfort to ourselves, and give the glory to God. God promised to give water, but they must open the ground. God's favours must be expected in the use of such means as are within our power, but still the power is only of God.