What is the meaning of "Azuvos Arei Aro'er"?
Rashi citing Targum Yonasan: It is an expression of destruction. This is like Ir'er, Tis'arer - cities of Aram will be undermined and overturned.
Rashi citing Eichah Rabasi, Pesichah 10]: Aro'er was of Eretz Yisrael - "V'Es Aro'er" (Bamidbar 32:34). Does he stand in (discuss) Damesek and announce about Aro'er?! Rather, there were markets in Damesek like the number of days in a solar year (365). Each has [its own] idolatry, and they serve it one day a year. Yisrael made them one collection, and serve all of them every day. Therefore, Aro'er is mentioned with Damesek.
Rashi: Because Retzin and Pekach ben Remalyahu joined together [against Yerushalayim], and the Navi prophesizes the fall of Damesek, it says that the cities of Aro'er, which were of Pekach, were already abandoned. Reuven and Gad were already exiled. Their land is always for flocks; now the flocks of Mo'av graze there. The kingdom of Pekach will decline, and also Shomron will be captured in the days of Hoshe'a. Then fortresses will cease from Efrayim, and kingship from Damesek; Retzin will be killed.
Radak: Perhaps there was a big city Aro'er in Aram. Small cities [nearby] are called on its name "Arei Aro'er." There was a city Aro'er of Mo'av that Yisrael conquered, and a city of this name in Aram - "Ad Aro'er Asher Al Pnei Rabah" (Yehoshua 13:25).
Malbim: By contrast, the cities of Aro'er of Efrayim will not be so desolate; refer to 17:2:2:2.
Why does it say "la'Adarim Tihyenah"?
Radak: They will be destroyed, and they will be a wilderness for Tzon.
Malbim: Flocks will graze there, "v'Ravtzu" (on a fixed basis, not just sporadically).
What do we learn from "v'Ein Macharid"?
Malbim: There will not be evil beasts or collapsed houses there.
