Adam Clarke on Psalms 83:15

So persecute them - In this and the two following verses we find several awful execrations; and all this seems to be done in reference to that ancient custom, "pouring execrations on an enemy previously to battle." Of this I have already given specimens in this work; and the reader is particularly requested to refer to the case of Balaam being hired by the king of Moab to curse Israel previously to his intended attack: see the note on Num 22:6, where the subject is treated at large.

This custom prevailed much among the Romans, and the ancient Druids of Britain. In all cases the priests were employed to utter the execrations, as they were supposed to have the greatest influence with the gods, in whose name the curses were uttered.