The capture of east Mosul was by no means an easy fight, taking about three months and involvi0ng pretty heavy casualties among Iraqi special forces. At the same time, officials concede that taking the city’s west side is going to be substantially harder.
“Victory tours” of captured ISIS-held houses in the city’s east underscore just how much planning ISIS has been doing for the defense of the city, with bomb making materials, instructions, and weapons manuals everywhere. Iraqi commanders concede that if ISIS hadn’t withdrawn so many of its fighters, the eastern battle could’ve dragged on a lot longer.
Western Mosul, by contrast, is the more populous half of the city, and the one ISIS had emphasized their defenses in from the start. ISIS fighters there also don’t have a lot of options in fleeing, with Shi’ite militias having taken the main roads of of town to block their escape.
On top of that, Iraq has very limited numbers of fighters really trained for urban combat, and that “Golden Brigade” suffered heavy casualties already in the fight. This force will be expected to do the heavy lifting again in the city’s west, with little time to resupply and no time to train new recruits.