Jamieson, Fausset and Brown on Isaiah 19:17

17. Judah ... 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 2Ki 23:29, where Josiah takes the field against Pharaoh-nechoh 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."

thereof--of Judah.

it--Egypt.