Moses Waging War on Midian

And the Lord said to me, do not be at enmity with Moab, neither contend with them in battle.

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What entered Moses’s mind, that God had to warn him
not to undertake a particular action? Did it enter
his mind to wage war with the Moabites without permission?
Rather, Moses reasoned an a fortiori inference by himself, saying:
And if with regard to the Midianites, who came only to help the
Moabites harm the Jewish people the Torah said:

Harass the Midianites and smite them.