If it had not been the LORD who was on our side, when men rose up against us:
If (it had not been) the Lord who was on our side. The repetition (cf. Psalms 124:1) marks that the Lord's interposition had been more than once.
When men rose up against us. However many and powerful be men, the believer, who has the Lord on his side, need not fear what man (a creature of earth) can do unto him (Psalms 56:11).