When Amalek attacked, Moses turned to Joshua with instructions that reveal what kind of army Israel would fight with. The Targum Pseudo-Jonathan records the order: "Choose such men as are strong in the precepts, and victorious in fight" (Exodus 17:9).
Note the order. Strong in the precepts comes first. Before physical prowess, Joshua is told to select men whose observance of the mitzvot was already sturdy. Muscle without merit would not hold the line against Esau's heir.
Moses then announces his own battle station: "Tomorrow I will stand, prepared with fasting, with the righteous fathers of the chiefs of the people, and the righteous mothers who are like the hills." The Targum pictures him on the hilltop flanked by the merit of Abraham, Isaac, Jacob, Sarah, Rebecca, Rachel, and Leah — the matriarchs "like the hills," an echo of Song of Songs 4:6. Their merit is the artillery.
And in his hand, "the rod with which the miracles have been wrought from before the Lord" — the same staff that had struck the Nile and split the sea. Two battlefields run in parallel. Joshua's men fight with sword and with Torah-observance below. Moses fights with prayer, fasting, and ancestral merit above. Israel wins only when both lines hold. The lesson: the most serious battles are fought on two planes at once.