Three [things] are here described: the preparation of the battle, the battle, and the outcome. And that [first] indeed, in verse 9, is set in three [parts]: in the appointing of one leader, in the choosing of the soldiers, in the promise of aid.
The leader is appointed by Moses — Joshua. For it is said to him by Moses, “Choose out men”; and [so] it is rendered to us in Hebrew. But in the Greek it is ἐπίλεξον σεαυτῷ ἄνδρας δυνατούς — “Choose for thyself mighty” (or “robust”) “men.” [That] these were younger [men], Philo says (book 1 On Moses). Yet our Latin [translator] expressed all these [things]. For while he says, absolutely, “Choose,” he sufficiently signifies [that] they are chosen at the discretion of the Leader himself, and [at] the judgment of the one commanding. And while he calls [them] men, he signifies [them] by no means weaklings and cowards, but strong and manly. For, as Tullius [Cicero] says in the Tusculans, “virtue [virtus] [is named] from man [vir].” R. Eliezer (ch. 44) makes Moses speak thus: “Choose for us men, sons of [good] fathers: not degenerate — men robust in strength and virtue, fearing God.”
Aid is promised: “To-morrow,” says Moses, “I will stand on the top of the hill, having the rod of God in my hand.”
The battle, therefore, [was] joined on the following day — of whose outcome [I shall speak] afterward. Now the Israelites had arms which they had stripped from the Egyptians drowned in the Red Sea, as Theodoret says (question 33 on Exodus), and Josephus (book 2) — although I would think [that] they also carried some out of Egypt. For if they carried off so much of gold, of silver, and of the other precious things which were afterward gathered into the tabernacle, why not also somewhat of arms?