Why does it say "Al Hasgiram"?
Radak: Also this was in Bayis Sheni. Also Tzor was close to Eretz Yisrael, and handed over fleers to Edom.
Malbim: This is the opposite of the Torah's command "Lo Sasgir Eved El Adonav" (Devarim 23:16).
What was "Bris Achim"?
Rashi: It was the Bris between Chiram and Shlomo 1 ? "va'Yichresu Vris Sheneihem" (Melachim I, 5:26). They called each other brothers ? "Mah he'Arim Asher Nasata Li Achi"? (ibid. 9:13)
Radak: The Bris that is proper to be between brothers (Yisrael and Edom). Tzor knew that Edom did not remember it, and were cruel to Yisrael; [even so, Tzor] handed Yisraelim over to Edom. Also Moshe sent to Edom "Ko Amar Achicha Yisrael" (Bamidbar 20:14).
Malbim: Yisrael relied on the Bris with Tzor from the days of David and Shlomo, and fled to Tzor, and Tzor handed them over to the Edomim.
