What is the meaning of "v'Haysah Admas Yehudah l'Egypt l'Chaga"?
Rashi: When the remnants of Egypt from Sancheriv's captives will hear of his fall in Eretz Yehudah without any human war 1 , they will know that there is Shechinah in Yisrael, and their Savior is strong, and they will fear Eretz Yehudah. Chaga is breakage and fear (Radak - trembling), like "Yachogu v'Yanuhu ka'Shikor" (Tehilim 107:27), "b'Chagvei ha'Sela" (Shir ha'Shirim 2:14).
Malbim: Whenever an Egyptian recalls Eretz Yehudah, he will think that it is a sign - perhaps Hashem counsels good or bad on him, and this caused him to remember Hashem's Kadosh land. The Egyptians were superstitious about everything.
Radak: They will then realize that also Egypt's fall to Sancheriv was due to Hashem's counsel.
What was Hashem's counsel for him?
Rashi: That Egypt will fall to Sancheriv, and Yehudah will be saved from him.
