Why Kill Midyan and Spare Moav?

God tell Moshe to fight the Midianites, but the sin of Ba’al Pe’or occurred with the daughters of Moav. Furthermore, God specifically says not to attack Moav! Ramban considers and critically evaluates varying approaches: Is the sin of planning and concocting worse than carrying something out- were the Midians merely using the Moavite women as their weapon of choice? Was Moav to be saved because of future righteous descendants? Was motivation a factor- Moav involved out of fear, and Midian out of malice? Is there a difference between attacking borders and attacking people’s souls? Is God’s exclusion of Moav related to this story, or to the covenant between Lot and Avraham?