Jamieson, Fausset and Brown on Ezekiel 29:21

21. In the evil only, not in the good, was Egypt to be parallel to Israel. The very downfall of Egypt will be the signal for the rise of Israel, because of God's covenant with the latter.

I cause the horn of ... Israel to bud--(Psa 132:17). I will cause its ancient glory to revive: an earnest of Israel's full glory under Messiah, the son of David (Lu 1:69). Even in Babylon an earnest was given of this in Daniel (Da 6:2) and Jeconiah (Jr 52:31).

I will give thee ... opening of ... mouth--When thy predictions shall have come to pass, thy words henceforth shall be more heeded (compare Eze 24:27).