I will bring a fear upon thee... — As in the case of Moab, there is the doom of exile for Ammon also, but the sentence of punishment is tempered with mercy, and there is to be a return from the seemingly hopeless captivity.
Related Commentaries of Jeremiah 49:5
Jeremiah 49:5-6
5 Behold, I will bring a fear upon thee, saith the Lord GOD of hosts, from all those that be about thee; and ye shall be driven out every man right forth; and none shall gather up him that wandereth.
6 And afterward I will bring again the captivity of the children of Ammon, saith the LORD.