exodus 17:9 And Moses said unto Joshua, Choose us out men, and go out, fight with Amalek: to morrow I will stand on the top of the hill with the rod of God in mine hand. - Free Bible Online

      9. Moses said unto Joshua--or, "Jesus" (Ac 7:45; Heb 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].

JFB.