"Then I said to you, 'Do not be in dread or afraid of them. The LORD your God who goes before you will himself fight for you, just as he did for you in Egypt before your eyes, and in the wilderness, where you have seen how the LORD your God carried you, as a man carries his son, all the way that you went until you came to this place.'"
Faith versus fear is one of the dominating struggles of the Christian life. Do I believe this promise of God to get me through? Or do I fear my circumstances and follow my own feet? Doubt and disobedience go hand in hand, it seems. And all the book of Deuteronomy is one long litany of fighting for faith in God and God's way, over doing it my way.
What is Moses's solution? Look back to all that God has already done for you and trust that He will keep on doing that. John Piper explains that it's using the past evidences of grace to file our fight ti trust in future grace. Fighting this enemy is no different than fighting that past one. Obeying this law will have the same effect as obeying that one. But oh how inclined my heart is to say "But this is NEW! God doesn't know how to handle this situation! I can't trust Him here!" oh foolish heart! "You have seen how the Lord your God has carried you" trust Him.