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, Psa_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 "Jud_3:11".