Deuteronomy 25:17-18 GNT

18 They had no fear of God, and so they attacked you from the rear when you were tired and exhausted, and killed all who were straggling behind.
19 So then, when the Lord your God has given you the land and made you safe from all your enemies who live around you, be sure to kill all the Amalekites, so that no one will remember them any longer. Do not forget!