Isaiah 19:17

And the land of Judah shall be a terror unto Egypt, every one that maketh mention thereof shall be afraid in himself, because of the counsel of the LORD of hosts, which he hath determined against it.

Judah shall be a terror unto Egypt - not by itself: but at this time Hezekiah was the active subordinate ally of Assyria in its invasion of Egypt under Sargon. Similarly to the alliance of Judah with Assyria here is , where Josiah takes the field against Pharaoh-necho of Egypt, probably as ally of Assyria against Egypt (G.V. Smith). Vitringa explains it, that Egypt in its calamities would remember that prophets of Judah had foretold them, and so Judah would be "a terror unto Egypt."

Every one that maketh mention thereof - of Judah.

Shall be afraid in himself, because of the counsel of the Lord of hosts, which he hath determined against it - against Egypt.