Charles Ellicott Commentary Jeremiah 50:18

"Therefore thus saith Jehovah of hosts, the God of Israel: Behold, I will punish the king of Babylon and his land, as I have punished the king of Assyria." — Jeremiah 50:18 (ASV)

As I have punished the king of Assyria. —Nineveh had fallen before Cyaxares and Nabopolassar, and Babylon was likewise to fall before Cyrus. The one judgment was the pledge and assurance of the other.