John Gill Commentary Jeremiah 51:56

"for the destroyer is come upon her, even upon Babylon, and her mighty men are taken, their bows are broken in pieces; for Jehovah is a God of recompenses, he will surely requite." — Jeremiah 51:56 (ASV)

Because the spoiler is come upon her, [even] upon Babylon , &c.] That is, Cyrus, with his army:

and her mighty men are taken ; unawares, by surprise:

everyone of their bows is broken ; they had no strength to withstand the enemy, and were obliged to yield at once; lay down their arms, and submit:

for the Lord God of recompences shall surely requite ; that God to whom vengeance belongs, and will recompense it; who is a God of justice and equity, the Judge of all the earth; he will render tribulation to them that trouble his; and requite his enemies and the enemies of his people, in a righteous manner, for all the evil they have done, as literal, so mystical Babylon; see (Revelation 18:6–8) .