IIRC the Israelites battled the Amaleks just before they reached the holy land.
And didn’t they wipe out all of them?
Most, but Haman, who launched the attempted genocide of the Jews in the Persian Empire that is covered in the book of Esther, was an Agagite (Esther 3:1). And that means he was, at least allegedly, descendant of King Agag (1 Samuel Ch. 15) Apparently there might have been exceptions to 1 Samuel 15:8, such as through daughters of Agag married off to foreign lords.