Why does it say "mi'Shamayim Hishmata Din"?
Radak: This is like "Esh v'Gafris Amtir Alav" (Yechezkel 38:22) - the judgment that You execute on them from Shamayim, You make it heard.
Malbim: When Sancheriv came against Tziyon, You sent an angel from Shamayim to judge him and do Mishpat of Your nation against the afflicter - "v'Hayah Ki Yivtza Hashem Es Kol Ma'asehu b'Har Tziyon uvi'Yerushalayim Efkod
What do we learn from "Eretz Yor'ah v'Shakatah"?
Shabbos 88a: At first, the land was afraid [lest Yisrael not accept the Torah, and the world will revert to emptiness] After Yisrael accepted the Torah, it was quiet.
Rashi: Initially, Eretz Yisrael feared Sancheriv. After Yeshayah prophesized about his punishment, and it was fulfilled, the land was quiet.
Radak: Because [You make Your judgment heard], the land of the nations feared Kel and ceased to fight against Yisrael.
