Albert Barnes Commentary Jeremiah 51:3

"Against [him that] bendeth let the archer bend his bow, and against [him that] lifteth himself up in his coat of mail: and spare ye not her young men; destroy ye utterly all her host." — Jeremiah 51:3 (ASV)

The man who bends the bow, and the heavy-armed soldier who vaunts himself in his coat of mail (Jeremiah 46:4, note), represent the Babylonians who defend the city.