Numbers 21:26

He had waged war. Why was it necessary for this to have been written? Because it is said, “do not oppress Moav,” and Cheshbon belonged to Moav. Scripture [therefore] writes, for us, that Sichon confiscated it from them, and it was purified for Israel through him.

From his hand. From his possesion.

For Heshbon was the city of Sihon, king of the Amorites; for he had beforetime made war with the King of Moab, and had taken all his country from his hand unto the Arnon.

FOR HESHBON WAS THE CITY OF SIHON THE KING OF THE AMORITES. That is to say, now when Israel waged battle against it, it was the city of Sihon the king of the Amorites, meaning to say the city of the royal residence, for he had fought against ‘melech harishon’ (the first king) of Moab, [meaning the one] who had ruled over them before any king reigned, or harishon [may mean] “the one before Balak,” who was the king of Moab at that time, and Sihon had taken all his land out of his hand, even unto the Arnon. Now Heshbon marked the border at which this conquest [of Sihon] began; therefore it was considered the Amorites’, and Israel was only prohibited [from capturing] that land of Moab and Ammon which was land that was in their possession at the time of the Divine command [not to capture any of the lands of Moab and Ammon]. And Scripture brings a proof that Heshbon was the city of Sihon, for those that speak in parables [as explained above in Verse 13] say, come ye to Heshbon etc., for when Sihon fought against the king of Moab, he captured at first the city of Heshbon, and the city was destroyed, and afterwards he rebuilt it as his royal abode. This is the meaning of [the phrase], who dwelleth in Heshbon. And similarly it is said in the Book of Joshua, Sihon king of the Amorites, who reigned in Heshbon. Thus those who spoke in parables would say to the Amorites: “Come to Heshbon, and settle therein; let the city of Sihon, over which he has become king, be built and established after its destruction.” And after Sihon had established himself in Heshbon, he assembled his army therein and captured from the border of Moab until Arnon, including Arnon itself. Arnon itself” is not found. Other scholars have also commented that the reading should be: “excluding Arnon itself.” This is undoubtedly correct, as Ramban himself expressly states above in Verse 13. Therefore they that speak in parables said, that a fire is gone out of Heshbon, a flame from the city of Sihon wherein he dwells; and it has devoured Ar of Moab, and the lords of the high places which belong to Arnon. For Sihon has captured from Moab unto Arnon, and all the high places of the land and the lords of the high places, this being similar [in meaning to]: even the ancient high places are ours in possession. He [Sihon] also took from the children of Ammon [the territory] from the Arnon to the Jabbok, as it is said in [the Book of] Joshua, And Moses gave unto the tribe of Gad etc. Jazer, and all the cities of Gilead, and half the land of the children of Ammon unto Aroer, this being the land which the king of Ammon claimed from Jephthah, as he said, Because Israel took away my land, when he came up out of Egypt, from the Arnon even unto the Jabbok, and unto the Jordan.

כי חשבון עיר סיחון מלך האמורי היא, the reason why we described all the towns of the Emorite as suburbs of Cheshbon is because Cheshbon had been the capital city of Sichon the King of the Emorite already before he had conquered any of the cities of Moav.

כי חשבון עיר סיחון, “for Cheshbon had been Sichon’s city originally, prior to his having conquered large sections of the lands of Moav and the Bney Ammon. His reputation as a mighty warrior was based on those campaigns. When the Israelites are described in verse 31 as settling in the land of the Emorite, this is only to inform us that this is where Sichon had originated as a king. This was long before Balak became king of what remained of the kingdom of Moav.