Because of the force - Rather, without force. Translate it: “The fugitives have stood, (i. e., halted) powerless in the shadow of Heshbon.” As Heshbon was the capital of the Ammonites, the sense is that the defeated Moabites looked to Ammon for protection.
But afire ... - Not only will Ammon refuse aid to Moab, but her ruin is to come forth from Heshbon. To show this Jeremiah has recourse to the old triumphal poetry of the Mosaic age (marginal reference).
The corner - i. e., of the beard ...”the crown of the head.” The fire of war consumes both far and near, both hair and beard, i. e., everything that it can singe and destroy.
The tumultuous ones - literally, “sons of the battle-shout,” the brave Moabite warriors.