Charles Ellicott 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)

You are my battle ax ... —Better, my mace. The axe is not found on Assyrian monuments as a weapon of war until a comparatively late period. It is a question who is addressed in this way—Babylon, or Cyrus as the destroyer of Babylon, or Israel. On the whole, the second seems the more probable answer. The hammer of the whole earth is broken by a mightier weapon than itself. (See Note on Jeremiah 50:23.)

With you I will break in pieces ... —The tense, in this and in the following, should be the present. The force of the verb is multiplied by the emphatic iteration. All obstacles are to be crushed in the victorious march of the conqueror.