When did Hashem leave Se'ir?
Rashi based on Targum Yonasan: It was at the time of Matan Torah - "v'Zarach mi'Se'ir Lamo" (Devarim 33:2).
Radak citing Ibn Ezra: This hints to wars that Hashem fought for Yisrael, and also "v'Zarach mi'Se'ir Lamo" (Devarim 33:2), "Eloka mi'Teiman 1 Yavo v'Kadosh me'Har Paran" (Chabakuk 3:3).
Malbim: "B'Tzeischa mi'Se'ir b'Tzadecha mi'Sedei Edom" refers to Yisrael's first war with the nations. They left Har Se'ir and fought with Sichon and Og 2 . Hashem went out in front of them and defeated them. With His power, they overpowered in all those wars.
Tosfos (Avodah Zarah 2b): Teiman is from Se'ir; it says "v'Chatu Giborecha Teiman Lema'an Yikares Ish me'Har Se'ir " (Ovadyah 1:9).
Malbim: The land next to Edom is called Sedei Edom, because it is the field around Edom. (Eretz Mo'av is between Edom and Sichon (11:18, 19)! Perhaps all of Amoni and Mo'av is called 'around Edom'; Sichon and Og took part, and part remained to Amon and Mo'av. - PF)
What do we learn from "Eretz Ra'ashah"?
Radak: All this hints to wars.
Mesilas Yesharim (24): Whenever the Shechinah appeared on a place, [the land] shook.
What water dripped from Shamayim?
Rashi: It is the dew of Techiyas ha'Mesim.
Malbim: This hints that also the Heavenly hosts melted [from fear].
QUESTIONS ON RASHI
Rashi writes that Hashem left Se'ir at the time of Matan Torah. What is the connection to here?
Rashi based on Targum Yonasan: It is hard to separate from Torah, and good to cling to it, for it was given amidst fear and strength. Therefore Yisrael were handed over to their enemies when they abandoned Torah, and when they volunteered to engage in it, they were saved.
