So, i.e. so suddenly, so surely, so effectually and irrecoverably. When he goeth forth in his might; when he first riseth, and so goeth on in his course, which he doth with great might, even as a strong man that runneth a race, Psalms 19:5, and so as no creature can stop or hinder him; even so irresistible let the people be. Forty years; how to be computed, See Poole on "Judges 3:11".
Related Commentaries of Judges 5:31
Judges 5:31
31 So let all thine enemies perish, O LORD: but let them that love him be as the sun when he goeth forth in his might. And the land had rest forty years.