Leningrad and Stalingrad were both destroyed because Russian (Soviet) forces attacked them to oust the Germans who were there  supposedly to liberate them from the dirty commies and to atone for the scurvy treatment that Germany received with the Treaty of Versailles after WW1. 
Aleppo was destroyed because the existing government of Bashar al-Assad attacked the city to oust what is essentially disparate outlaw groups of fundamentalist Muslims, opportunists, thrill seekers, criminal sadists, and violent nut jobs who were supposedly there because the Syrian government was too harsh in their attempts in enforcing secular law to modernize the country.