What will arouse the chiefs of Yehudah to say so?
Rashi: They will see the other besiegers stricken with a plague, and they themselves are saved.
Malbim #1: Mashi'ach ben Yosef will rise then, and Bnei Yehudah will be strengthened then.
Malbim #2: According to Chazal, that Gog will come to Eretz Yisrael three times, the first time, each horse will be blinded 1 . And the last [two] times, Bnei Yehudah will be strengthened.
Malbim (4) explained that the besiegers' horses will be stricken with Timahon, and Yishmaelim's horses with Ivaron! This requires investigation. (PF)
What is the meaning of "Amtzah Li Yoshevei Yerushalayim"?
Rashi: Residents of Yerushalayim, who know that we are coerced to come against them, should invigorate themselves in prayer to Hashem for us. Radak ? Amtzah is a noun, with the grammatical form of Olah, Kavsah and Shalvah.
Malbim: They will say in their hearts that Yoshevei Yerushalayim are Tzadikim, and they will give to them strength via their virtue and their Bitachon in Hashem.
