Radak: When Hashem strikes Machaneh Ashur, people will see that they fell without a sword. Hashem did so. They will recognize that Hashem is Elokim, and there is nothing else. The idols that their hands made, this is a sin and a great mistake. They do not help or save, for they are nothing. They did not save Arei Yehudah. Yerushalayim was saved, because it strengthened itself in Hashem and prayed to Him to save them from Melech Ashur.