So persecute them - So pursue them; so follow them up. The word "persecute" is now used in a somewhat different sense, as denoting pain or suffering inflicted on account of religious opinion. It means here simply to pursue.
With thy tempest - With the expressions of thy displeasure; with punishment which may be compared with the fury of a storm.
And make them afraid with thy storm - Or, Make them afraid, terrify them, so that they will flee away. As all that is here sought by prayer is what people endeavor to do when an enemy invades their country - as they make arrangements for repelling those enemies, and overthrowing them, and as they feel that it is right to do so - there is no impropriety in making this the subject of prayer to God. What it is right for men to attempt, it is right to pray for; what it would be right for them to do if they had the power, it is right to ask God to accomplish; what is free from malignity in the act, and in the design, may be free from malignity in the desire and the prayer; and if men can carry with them the idea that what they are endeavoring to do is right, whether as magistrates, judges, rulers, defenders of their country, or as private men, they will have very little difficulty in regard to the so-called "imprecatory psalms." See this subject treated in the General Introduction

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 Numbers 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.

So persecute them with thy tempest,.... Pursue them with thy fury, follow them with thy vengeance; cause it to fall upon them like a mighty tempest:
and make them afraid with thy storm; God has his storms and tempests of wrath and vengeance, which he sometimes causes to fall upon wicked men in this life, to their inexpressible terror, and with which he takes them out of this world; and he has still more horrible ones to rain upon them hereafter: see Job 27:20.