But what appeared to be a comprehensive plan that would see much of Iraq’s military rebuilt after it melted away in the face of Isil last June, quickly began to unravel.
With fewer than two months to go before the operation was due to begin, the US admitted it has so far only trained a fraction of the required soldiers.
An official at US Central Command said coalition troops have so far only trained “approximately 3,400 Iraqi troops”.
Many of these have received only the initial “bootcamp” training; a six-to-eight week course for new recruits designed to teach them the very basics of combat.
Demonstrators chant pro-Islamic State group, slogans as they carry the group’s flags in front of the provincial government headquarters in Mosul in June 2014 (AP)
The US is in the process of deploying 3,000 military advisors to as many as eight training locations, as part of a $1.2 billion (£780 million) American train-and-equip programme to help the Iraqi army.
But the programmes have gotten off to a difficult start. In Camp Taji, one of the main training centres, recruits were, as recently as January, being trained mostly without weapons because the bulk of their equipment had been entangled in “Iraqi bureaucracy”, according to the Washington Post.
When asked how the US intended to launch the operation given the current dearth of soldiers, the spokesman for Central Command said the assault could be delayed.
“May is just an initial goal. If senior leaders do not feel that by then they are in a position where they can be successful, they can change it.”
The Iraqi and Kurdish security forces the US-led coalition is working with to prepare for the Mosul offensive also expressed surprise and dismay at the plan.
An Iraqi security source, who asked not to be named as he did not have permission to speak to journalists, said he found it “very weird” that the US should make such a statement, adding that May was a wholly unrealistic goal.
“His estimate is innacurate,” he said. “Before reaching Mosul we have to clear the towns of Kirkuk, Diala and Tikrit”.
Both the Iraqi and the Kurdish security sources said the US had also dramatically underestimated the number of Isil fighters in the city.
Instead of 1,000 to 2,000 Isil fighters in Mosul, as the US has claimed, they put the figure closer to 10,000.
The success of the offensive would likely depend on the participation of Kurdish Peshmerga forces – one of the only fighting groups left in the north of the country after the Iraqi military fled.
But the Kurdish security source said it was far from given that they would participate in the attack. Kurdish leaders have made it clear they have little interest in taking Mosul , a city outside the territory of an aspirational Kurdish state.