Kurdish, Iraqi and US officials have reported slow but steady progress in the first day of the announced battle to retake the last Iraqi stronghold of Mosul from Isis.
The Pentagon said yesterday it was "very confident" in the success of the US-backed operation, which will have major repercussions for Iraq's future, as well as the legacy of US President Barack Obama.
If successful, Operation Inherent Resolve will effectively spell defeat for Isis in the country - although unlike the militants' efforts to retain control in other cities, such as Fallujah, experts expect Isis will not give up their 'second city' without a fight, putting up to 1.5m civilians in danger.
Long convoys of around 30,000 Iraqi and Kurdish troops mobilised around dawn yesterday, accompanied by the roar of Western coalition jets and helicopters which dropped flares.
By sundown, Kurdish forces said they had made slow progress to the east of Mosul, retaking seven villages mostly deserted of civilians in the Ninevah plain, a corridor leading to the city.
Isis claimed no fewer than 12 suicide bomb attacks, designed to slow the Kurdish troops' advance.
Iraqi intelligence reports suggested that Isis fighters in the region were fleeing back to neighbouring Syria with their families, an Iraqi army commander said, adding that the offensive was "going very well."
Iraqi Prime Minister Haider al-Abadi appeared on state television in the early hours to announce that the long-awaited attempt to free the city had begun in what is set to be the militants' toughest military test to date.
"These forces that are liberating you today, they have one goal in Mosul, which is to get rid of Daesh [Isis] and to secure your dignity," Mr Abadi said, addressing residents who have suffered under the group's interpretation of Sharia law for more than two years.
Mosul, Iraq's second largest city, was home to two million people before it was captured by Isis in the group's blitz-like expansion across Iraq in the summer of 2014. Aid organisations are worried for the welfare of the estimated 1.5m left trapped in the city, which is likely to see prolonged and heavy fighting.
Around 8,000 Isis fighters are thought to be well prepared for battle, building extensive tunnel networks, trenches to be filled with burning oil, and rigging roads and bridges with explosives.
Around 200,000 people are expected to flee the first few days of the offensive, despite repeated requests from the Iraqi government for civilians to stay indoors and place white flags on the roofs of their homes.
While the military promised that details of planned humanitarian corridors out of Mosul to nearby camps would be provided, several international agencies have raised concerns that the military seems more intent in locking the city down to prevent anyone from leaving. "Unless safe routes to escape the fighting are established, many families will have no choice but to stay and risk being killed by crossfire or bombardment, trapped beyond the reach of humanitarian aid with little food or medical care," Aram Shakaram, Save the Children's Deputy Country Director in Iraq, said.
"Those that try to flee will be forced to navigate a city ringed with booby traps, snipers and hidden landmines. Without immediate action to ensure people can flee safely, we are likely to see bloodshed of civilians on a massive scale."
Even if residents manage to leave Mosul, funding shortages mean the UN and partner agencies are under equipped to deal with even half the number of people expected to need emergency aid in the man-made crisis.
The loss of Mosul, from where Isis leader Abu Bakr al-Baghdadi announced the creation of the group's so-called caliphate, would be a "massive blow" to Isis' ideological stance, Dr Natasha Underhill of Nottingham Trent University said.
"Isis is no longer the powerhouse that it once appeared and, is in fact, struggling not only to gain support, but to keep the support in place that it currently has," she said. "It may be the beginning of the end for Isis as we know them."