They that fled stood under the shadow of Heshbon because of the force: but a fire shall come forth out of Heshbon, and a flame from the midst of Sihon, and shall devour the corner of Moab, and the crown of the head of the tumultuous ones. They that fled stood under the shadow of Heshbon - Heshbon being a fortified place, they who were worsted in the fight fled to it, and rallied under its walls; but, instead of safety, they found themselves disappointed, betrayed, and ruined. See Jeremiah 48:2 (note), and the note there.
But a fire shall come forth out of Heshbon - Jeremiah has borrowed this part of his discourse from an ancient poet quoted by Moses, Numbers 21:28 (note); where see the notes.
The crown of the head - The choicest persons of the whole nation.