Jamieson, Fausset and Brown on Exodus 17:9

9. Moses said unto Joshua--or, "Jesus" (Ac 7:45; He 4:8). This is the earliest notice of a young warrior destined to act a prominent part in the history of Israel. He went with a number of picked men. There is not here a wide open plain on which the battle took place, as according to the rules of modern warfare. The Amalekites were a nomadic tribe, making an irregular attack on a multitude probably not better trained than themselves, and for such a conflict the low hills and open country around this wady would afford ample space [Robinson].