Albert Barnes Commentary 2 Kings 19:19

"Now therefore, O Jehovah our God, save thou us, I beseech thee, out of his hand, that all the kingdoms of the earth may know that thou Jehovah art God alone." — 2 Kings 19:19 (ASV)

If the mighty army of the great Assyrian king were successfully defied by a minor king like Hezekiah, it would force the surrounding nations to confess that the escape was because of the protecting hand of Yahweh. They would thus be taught, in spite of themselves, that He, and He alone, was the true God.