Here is one approach that I found interesting:
As noted earlier, many reasons are cited for Joshua's having been chosen as military leader of Israel. However, the homilist R. Zevi Hirsch of Vilna in his Maggid Mesharim suggests that the true reason for Joshua's selection was due to Moses' ethical sensitivities. Moses was concerned that the Israelites might adopt Amalekite tactics in their battle plan against Amalek! R. Zevi points out how it is common practice for warring armies to carefully study and emulate on another's military tactics and procedures:
"Behold, the Torah has described the nature of the weapons of Amalek, i.e., a lack of reverence for the Lord. More horrible than all else are the evil attributes and abominable cruelties perpetrated by man in this state of faithlessness. For, if one has no fear of any divine power, there will be no restraints upon his lusts, and he will do whatever his heart desires, for he reasons that there is neither judge nor judgment... only a reverence for the Lord can possible restrain and control one's behavior...
But, what if Israel should choose to emulate Amalek and employ his tactics? Why should we not practice deceptions, deceits... robbery, murder, all possible worldly abominations? Since they are his weapons of war, we, too, shall employ them!"
It is for this reason that Moses chose Joshua to lead the Israelites into battle, confident that Joshua would not stoop to employing Amalek's reprehensible military stratagems. The only proper weapons of Israel are "fear of the Lord" and "ethical commitments." Only by means of sincere faith and compassionate living can Israel hope to triumph.
Maggid Mesharim presents an interesting theory as to why this "ethical approach" is bound to succeed:
"When the nations of the world perceive Israel practicing righteousness, integrity, kindness and compassion toward all peoples, they will perforce assent with an "amen," and proclaim, 'There is no nation as righteous as Israel.'"