Interpreters are here divided, whether by thou or thee in this and the following verses to understand Cyrus, whom God made use of to destroy Babylon and many other places, or Babylon. Our translators understand it of Cyrus, and therefore speak of the future tense, will I. The Hebrew text will not resolve us; I rather incline to interpret it of Babylon, as indeed the most do, and so it should be, Thou hast been, and art, for that is the sense; Cyrus and Darius were not yet in being. God had made use of Babylon like a hammer or battle-axe to break many nations in pieces.
Related Commentaries of Jeremiah 51:20
Jeremiah 51:20
20 Thou art my battle axe and weapons of war: for with thee will I break in pieces the nations, and with thee will I destroy kingdoms;