Charles Ellicott Commentary Psalms 79:10

"Wherefore should the nations say, Where is their God? Let the avenging of the blood of thy servants which is shed Be known among the nations in our sight." — Psalms 79:10 (ASV)

Wherefore. —Taken from Joel 2:17.

Let him be known. —Better, Let it be known, i.e., where God is. Let the answer to the question be given in vengeance, and let us see it.