IIRC Avraham was praised for arguing with G-d about the fate of pious Sodomites asking G-d to spare their fate. G-d spared Lot and his family but the two cities were destroyed. Some speculate whether this was a metaphor for Lot's decision. Whether or not this is the case, the question remains.
I was thinking about a more general question about Moses wars, but decided to focus on the war on Midianites. What would happen if he [Moshe] were to follow Avraham's example [tradition]? He ought to at least ask G-d why not - Even though the Moabites were the main reason for this war G-d spared them all because of the potential that will come from them. Mosh Rabbeinu could have asked whether there is no such potential in Midianite.
Why didn't he?