If it had not been the Lord who was on our side - Unless it was Yahweh who was with us. The idea is, that someone had been with them, and had delivered them, and that such was the nature of the interposition that it could be ascribed to no one but Yahweh. It bore unmistakeable evidence that it was his work. The deliverance was of such a kind that it could have been accomplished by him only. Such things often occur in life, when the intervention in our behalf is so remarkable that we can ascribe it to no one else but God.
Now may Israel say - May well and truly say. The danger was so great, their helplessness was so manifest, and the deliverance was so clearly the work of God, that it was proper to say that if this had not occurred, ruin would have been inevitable and entire.