Remember what the Amalekites did to you on your way out of Egypt.
You were tired and worn out. They met you on your journey. They attacked everyone who was lagging behind. They didn't have any respect for God.
The LORD your God will give you peace and rest from all of the enemies who are around you. He'll do it in the land he's giving you to take over as your very own. Then you will wipe out the memory of the Amalekites from the earth. Don't forget to do it!