במדבר כא:כח פירוש: רש״י, רשב״ם, רבנו בחיי וSiftei Chakhamim

כי אש יצאה מחשבון FOR A FIRE WILL GO FORTH FROM HESHBON after Sihon captures it,

ער מואב, the capital of the land of Moav was called Or.

כי אש יצאה מחשבון, ”for a fire has come forth from Cheshbon.” This is an allusion to Moabites who had dwelled in Chesbon and commenced a revolution against the local king.