Albert Barnes Commentary Psalms 9:20

"Put them in fear, O Jehovah: Let the nations know themselves to be but men. Selah" — Psalms 9:20 (ASV)

Put them in fear, O Lord — From this, it is evident that the psalmist's enemies were bold, daring, confident in their own strength, and in the belief that they would succeed. He prays, therefore, that these bold and daring invaders of others' rights might be made to stand in awe and tremble before the great and terrible majesty of God. He desires that they might thus have just views of themselves and see how weak and feeble they were compared to Him.

That the nations may know — The nations particularly referred to in this psalm as arrayed against the writer.

Themselves to be but men — The prayer is that they may see themselves as they are: poor, feeble creatures, as nothing compared to God. It means that instead of their pride, self-confidence, and belief that they can accomplish any purpose they choose, they would see that they are not like God, but are frail and feeble mortals. The psalmist seems to have believed that if they understood this, they would be humbled and would desist from their purposes. Therefore, he prays that God would intervene and show them precisely what they were. If men understood this, they would not dare to array themselves against their Maker.