Jamieson, Fausset and Brown on Jeremiah 51:3

3. Against him that bendeth--namely, the bow; that is, the Babylonian archer.

let the archer bend--that is, the Persian archer (Jr 50:4). The Chaldean version and Jerome, by changing the vowel points, read, "Let not him (the Babylonian) who bendeth his bow bend it." But the close of the verse is addressed to the Median invaders; therefore it is more likely that the first part of the verse is addressed to them, as in English Version, not to the Babylonians, to warn them against resistance as vain, as in the Chaldean version. The word "bend" is thrice repeated: "Against him that bendeth let him that bendeth bend," to imply the utmost straining of the bow.