Then came Amalek. The bulk of the Amalekites would have been passing the spring in the lower plains, where herbage is abundant after the early rains, while later in the year it dries up. They would hear of the threatened occupation of their precious summer pastures by the vast host of the Hebrews, and would seek to prevent it by blocking the way.
Hence they are said to have "come"—i.e; to have marched into a position where they were not previously, though it was one situated within their country. We must remember that they were nomads. And fought with Israel For the nature of the fighting on the first day, see Deuteronomy 25:18; by which it appears that the original attack was made on the rear of the long column, and was successful.
The Amalekites "smote the hindmost" of the Israelites, "even all that were feeble behind them, when they were faint and weary."