John Calvin Commentary Numbers 21:26

"For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto the Arnon." — Numbers 21:26 (ASV)

For Heshbon was the city of Sihon (Numbers 21:26). It is for good reason that Moses relates how the country near Heshbon had fallen into the hands of the Amorites. A long time later, this was used as a pretext for war by the Ammonites when they saw that the people were in a vulnerable state.

Therefore, in the time of Jephthah, the Ammonites, having gathered a large army, launched an invasion. They used this as their excuse: that they took up arms to recover what was theirs, from the Arnon to the Jabbok, and as far as the Jordan.

Consequently, God intended for it to be testified in the sacred records, as Jephthah then replied to the Ammonites, that this part of the land was taken from King Sihon while the children of Israel were marching peacefully through the borders of the Ammonites.

Moses, therefore, intentionally inserted into these official records, so to speak, what had previously happened, in order to validate the people's right. This record showed that the Amorites had dominion over that part of the country without interference from the Ammonites, and there was no doubt that the Amorites had secure and peaceful possession of it.

Therefore, it follows that the land passed to the Israelites, so there were no grounds for the Ammonites, three hundred years later, to reclaim what they had lost and abandoned so long ago.

And, so that posterity would know there was no lack of clarity about the matter at that time, Moses records an ancient canticle. From this canticle, it is clear that the Ammonites were so completely overcome that their enemies triumphed magnificently over them and cut off all hope of their restoration.

Here, however, the question arises: why did the king of Ammon, rather than the king of Moab, initiate that war? For we clearly gather from the song that the land was originally taken from the Moabites.

But for men who are intent on plunder and robbery, it is enough to claim any trivial pretext and often to boast in the rights of others.

Undoubtedly, a report remained that the Amorites had been driven out of their territories,¹³¹ which they had obtained by force of arms. The Ammonites silently ignored what had been forgotten over many ages and asserted this false claim: that although the Israelites had conquered the Amorites, their victory still did not give them the right to occupy what the Amorites had unjustly and forcibly held.

For this reason, Moses inserted the account he gives here.

¹³¹ “Par les enfans d’Israel;” by the children of Israel. — Fr..