What does Psalm 137:8 mean?

The psalmist is calling on the Lord to bring vengeance on those who destroyed Jerusalem (Psalm 137:7). The Edomites apparently helped Babylon capture Jews and desolate the city (Obadiah 1:10–14). For this, the writer has prayed for justice. Other Scriptures noted the idea of Edom suffering the very things they inflicted on Israel (Obadiah 1:15, 18). Here, the same idea is applied to Babylon.

When Jerusalem fell to Babylonian invaders, the carnage was horrific (2 Chronicles 36:17–21; 2 Kings 25:5–7). Among the atrocities was the vicious murder of innocents, including children. This was common in ancient warfare (2 Kings 8:12; Isaiah 13:16; Hosea 10:14). As the psalmist vows to remember what has happened (Psalm 137:6), he prays for Babylon to experience these exact same harms (Psalm 137:9). This is a bitter, vengeful prayer; yet it corresponds to the Lord's predictions about what would happen to the nation that abused Israel (Jeremiah 25:12; Isaiah 47:1–3; 50—51).