וַיִּשְׁלָחֲךָ֥ יְהוָ֖ה בְּדָ֑רֶךְ וַיֹּ֗אמֶר לֵ֣ךְ וְהַחֲרַמְתָּ֞ה אֶת־הַֽחַטָּאִים֙ אֶת־עֲמָלֵ֔ק וְנִלְחַמְתָּ֣ ב֔וֹ עַ֥ד כַּלּוֹתָ֖ם אֹתָֽם׃
and the LORD sent thee on a journey, and said: Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed.
Read halakhic rulings (Jewish law) on I Samuel 15:18: and classical halakhic sources.