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While elements of the Iraqi Army have gradually been able to evict ISIS from Iraq's smaller towns and cities, the challenge of liberating Mosul was of a whole different order of magnitude, it’s a vast metropolis that had millions of inhabitants, enormous sectarian and ethnic complexity, and was the historical bastion of Sunni [ Muslim ] revanchism in Iraq. The fight was always going to be toughest in Mosul, which is why it was left for last.