John Gill Commentary Jeremiah 48:20

"Moab is put to shame; for it is broken down: wail and cry; tell ye it by the Arnon, that Moab is laid waste." — Jeremiah 48:20 (ASV)

Moab is confounded, for it is broken down This is the answer returned, by those that had escaped and were fleeing, to those who inquired of them; who report that the whole country of Moab was in the utmost confusion and consternation; not being able to stand before the enemy, who broke down and destroyed all that was in his way.

And therefore calls upon them to howl and cry; because of the general ruin at the nation, and who must expect themselves to share the same fate; and therefore should prepare themselves and their neighbours for it, as follows:

Tell you it in Arnon, that Moab is spoiled; the country of Arnon, so called from a river of that name, on the banks of which Aroer was situated; the inhabitants of which are desired to spread it all over that part of the country, that Moab was utterly ruined by the Chaldean army; the particulars of which follow: