Albert Barnes Commentary Psalms 137:8

"O daughter of Babylon, that art to be destroyed, Happy shall he be, that rewardeth thee As thou hast served us." — Psalms 137:8 (ASV)

O daughter of Babylon - That is, Babylon itself; the city of Babylon. On the word “daughter” as it is used here, see the notes at (Isaiah 1:8).

Who art to be destroyed - Certainly to be destroyed; of whose destruction there are fixed and absolute prophecies. See the notes at (Isaiah 13:19–22).

Happy shall he be that rewardeth thee as thou hast served us - Margin, that recompenseth unto thee thy deed which thou didst to us. Literally, “Happy shall he be who shall repay to thee the recompence which thou hast recompensed unto us.”

The idea is, who shall repay you for your treatment of us; or, as we should say in common language, “Who shall pay you back?” That is, he will be esteemed a fortunate man who is made the instrument of inflicting deserved punishment on a city so guilty and so cruel. He will acquire fame and honor by doing it; his name will be made known abroad and perpetuated among people.

In fact, the name of Cyrus, who conquered Babylon, is among the names of the most celebrated of conquerors; and the manner in which he took Babylon and overthrew the government and kingdom, has given him a most eminent place among successful princes and conquerors.