The Triumph Song of the Amorites (Numbers 21:27-29).
‘That is the reason that they who speak in proverbs say,
“Come you to Heshbon;
Let the city of Sihon be built and established,
For a fire is gone out of Heshbon,
A flame from the city of Sihon,
It has devoured Ar (or ‘the cities') of Moab,
The lords of the high places of the Arnon.
Woe to you, Moab!
You are undone, O people of Chemosh,
He has given his sons as fugitives,
And his daughters into captivity,
To Sihon king of the Amorites.
In vivid language the defeat of Moab by Sihon is described. First he captured Heshbon, which would become his royal city, and fortified it (established it'). Then he moved down like a flame and a fire as far as ‘Ar, a city of Moab and defeated the border lords, seizing a number of captive slaves. And although Chemosh was the god of Moab he was able to do nothing about it. Indeed he surrendered them to the opposing forces.
The point behind the song was not only to display Sihon's belligerency, but also to expose Chemosh's helplessness. But now Israel had defeated Sihon. That proved that Yahweh was Lord over all. And as Chemosh had surrendered the land, Moab no longer had a claim on it.
“Ar was to the south of the Arnon but may simply be named as the nearest city to the border, being wasted but not retained. However, in early Hebrew ‘r could probably also signify ‘are (‘cities”) and that may be the rendering here.