Deuteronomy 25:19

Therefore it shall be, when the Lord thy God hath given thee rest from all thine enemies round about,.... Not only when they had subdued the Canaanites, and got possession of their land, but when they were clear and free from all their neighbouring nations, Moabites, Midianites, Edomites, Ammonites, and Philistines; wherefore it may be observed, that this did not take place, as not immediately after the conquest of Canaan, so neither in the times of the judges, when they were harassed frequently by their neighbours, and not until the times of Saul, the first king of Israel:

in the land which the Lord thy God giveth thee [for] an inheritance to possess it; the sense is, when they were in the full possession of the land given them by the Lord, as an inheritance to be enjoyed by them and theirs; and were at an entire rest from all enemies, and so had their hands at liberty to employ against Amalek:

[that] thou shalt blot out the remembrance of Amalek from under heaven: that is, utterly destroy them, so that there should be none left of them any where, to put in mind that there ever were such a people on earth; men, women, children, cattle of all sorts, were to be destroyed, and nothing left that belonged unto them; that it might not be said this beast was Amalek's, as Jarchi, and to the same purpose Aben Ezra; see the order for this renewed, and the accomplishment of it, at least in part, 1 Samuel 15:2, c.

thou shall not forget [it] neither the unkindness of Amalek, nor this order to destroy him. The Targum of Jonathan adds,

"and even in the days of the King Messiah it shall not be forgotten.''