Charles Ellicott Commentary 1 Samuel 15:2

"Thus saith Jehovah of hosts, I have marked that which Amalek did to Israel, how he set himself against him in the way, when he came up out of Egypt." — 1 Samuel 15:2 (ASV)

That which Amalek did to Israel. —The Amalekites were a fierce, untameable race of wanderers, who roamed at large through those deserts that lie between Southern Judea and the Egyptian frontier. They were descended from Esau’s grandson, Amalek.

Not long after the exodus from Egypt, they attacked and cruelly harassed the almost defenceless rearguard of Israel in the desert of Rephidim. They were then, at the prayer of Moses, defeated by Joshua; but, for this cowardly, unprovoked attack, solemnly doomed to destruction.

In the prophecy of Balaam they are alluded to as the first of the nations who opposed the Lord’s people. During the stormy ages that followed, the hand of Amalek seems to have been constantly lifted against Israel, and we read of them perpetually as allied with their relentless foes.