Albert Barnes Commentary Jeremiah 51:20

"Thou art my battle-axe and weapons of war: and with thee will I break in pieces the nations; and with thee will I destroy kingdoms;" — Jeremiah 51:20 (ASV)

Or, You are my maul, weapons of war, etc. The maul or mace (Proverbs 25:18) only differs from the hammer (Jeremiah 50:23) in being used for warlike purposes.

Omit the “will” in “will I break.” The crushing of the nations was going on at the time when the prophet wrote. Most commentators consider that Babylon was the mace of God.