Smote the Amalekites - This war is mentioned in the following chapter.

And he gathered an host, and smote the (t) Amalekites, and delivered Israel out of the hands of them that spoiled them.
(t) As the Lord had commanded, (Deuteronomy 25:17).

And he gathered an host,.... A large army; for after the battle with the Ammonites he disbanded his army, and sent them home, retaining only 3000 men, and these deserted him to six hundred, which were all the men he had with him, when he fought last with the Philistines; but now, finding he had enemies on every side of him, he gathered a numerous host to defend his country against them, and particularly to attack the people next mentioned:
and he smote the Amalekites; a people that Israel, by the law of God, were bound to destroy, and blot out their name; a particular account of his expedition against them is given in the following chapter:
and delivered Israel out of the hands of them that spoiled them; the nations before mentioned, Moabites, Ammonites, Edomites, Syrians, and Philistines.

"And he acquired power;" חיל עשׂה (as in Numbers 24:18) does not merely signify he proved himself brave, or he formed an army, but denotes the development and unfolding of power in various respects. Here it relates more particularly to the development of strength in the war against Amalek, by virtue of which Saul smote this arch-enemy of Israel, and put an end to their depredations. This war is described more fully in 1 Samuel 15, on account of its consequences in relation to Saul's own sovereignty.