The toll has been brutal, with thousands of Iraqi forces killed.
But since anti-IS operations began in Iraq and Syria in 2014, only 11 US troops have been killed.
The US military is trying a similar strategy with Afghan security forces in their fight against a resurgent Taliban.
– ‘Only horse we had’ –
For Brian McKeon, a senior Pentagon policy official at the end of the Obama administration, the strategy worked, though not as quickly as had been hoped. The battle for Mosul first began on October 16, 2016.
Once the decision was made to work “by with and through partners… it was the only horse that we had to ride,” McKeon said.
“It has taken longer than might have been assessed at the beginning but that’s not really unusual. No plan survives first contact and you never know where it’s going to go, given the large number of variables in a war.”
The strategy of supporting a proxy army will become increasingly important as the United States shies away from full-on deployments.
For John Spencer, a scholar at the Modern War Institute at West Point, the fight for Mosul has been “the biggest modern case study foreshadowing what (urban) war is going to be like in the future.”
“It’s kind of the ultimate end of that scale where you build an army, a police force, and a counterterrorism force that are capable of fighting, and you send only a few hundred troops and air support to help,” he said.
The United States is employing the same tactic in Syria, where commandos have trained a Kurdish-Arab alliance called the Syrian Democratic Forces to tackle IS.
Pentagon chief Jim Mattis calls it the “era of frequent skirmishing,” when local forces will be key in repelling non-state groups such as IS.
“We will do it by, with and through other nations,” he said in a recent interview with CBS News.
Though the fight against IS isn’t over, Canadian Brigadier General Dave Anderson, who oversees the training of local forces for the US-led coalition, said he was confident Iraqi forces would never again face a rout such as 2014.
“I had a very senior Iraqi tell me that, ‘We’re an ancient society and a brand new country; born in 2012, we’ve had a near-death experience in 2014.’ That’s actually a good lens through which to look at it,” he said.