Write this … in a book. The original has, "Write this in the book." It is clear that a book already existed, in which Moses entered events of interest, and that now he was divinely commanded to record in it the great victory over Amalek, and the threat uttered against them. The record was to be for a memorial—
1. that the victory itself might be held in remembrance through all future ages, as a very signal instance of God's mercy; and
2. that when the fulfilment of the threat came (1 Chronicles 4:43), God might have his due honour, and his name be glorified. Rehearse it in the ears of Joshua. "Hand down," i.e; to thy successor, Joshua, the tradition of perpetual hostility with Amalek, and the memory of the promise now made, that the whole nation shall be utterly blotted out from under heaven. (Compare Deuteronomy 25:19.) The special sin of Amalek was,
1. That he attacked God's people, not fearing God (Deuteronomy 25:18);
2. That he had no compassion on his own kindred: and
3. That he fell on them when they were already suffering affliction, and were "feeble, and faint and weary" (ib,)