Deuteronomy 32:42
I will make mine arrows drunk with blood, and my sword shall devour flesh; and that with the blood of the slain and of the captives, from the beginning of revenges upon the enemy. I will make mine arrows drunk with blood, and my sword shall devour flesh; and that with the blood of the slain and of the captives, from the beginning of revenges upon the enemy.
This verse belongs to Moses' song in Deuteronomy 32:1-43, which is both a historical and a prophetic account recounting God's dealings with His people. As prophecy it foreshadows the final victory of the faithful when Christ returns in power and glory. God, in Christ, commanded Moses to frame the song out of His dealings with ancient Israel and to teach it to Israel.
The song was given to preserve the memory of God's dealings with His people and to keep the history of the past from being forgotten by passing it on to children. Parents were to teach it to their children and to repeat the whole story when children inquired about it. It keeps in mind God's providential dealings and His marked goodness and mercy in His care and deliverance of His people, standing as one of the waymarks He frequently sets up. It shows that God's dealings should often be repeated with praise and thanksgiving.