This is the big one – the battle to retake the Iraqi metropolis of Mosul began Monday as thousands of Iraqi and Kurdish forces, supported by U.S. jet fighters, advanced toward the city that has been held by extremist Islamic State fighters since June, 2014.
Mosul was declared the capital of the IS caliphate by the group's leader, Abu Bakr al-Baghdadi, and it seems certain that, though IS fighters have been in retreat throughout Iraq this year, it is here that the militants will stage their greatest defence.
Mosul is much larger than any of the Iraqi cities such as Fallujah that recently have been recaptured, and the competing agendas of ethnic and religious groups make the battle for Mosul much more problematic.
These are the five hurdles that must be overcome.
Islamic State forces are well-entrenched
During 28 months of occupation, Islamic State has been preparing for this day. Its heavy weapons will have been dug in, the U.S. tanks it captured when overrunning Iraqi army posts in 2014 will be under cover but ready to join the fight at different points, roads and buildings will have been booby trapped and snipers will be in position. The closer the Iraqi forces get to the city itself, the slower the progress will be.
Complicating matters further, Mosul's civilian population will act – willingly or unwillingly – as human shields for IS forces. With no clear path out of the city for families wanting to flee, tens of thousands are in danger from crossfire, warns Aleksandar Milutinovic of the International Rescue Committee.
Civilians attempting to escape "will have little choice but to take their lives into their own hands and pray that they are able to avoid snipers, land mines, booby traps and other explosives."
Refugee numbers could be overwhelming
More than a million civilians remain under IS occupation in Mosul and its adjacent villages; hundreds of thousands of them are expected to try to escape. Such numbers will inundate the advancing Iraqi and Kurdish forces.
The United Nations emergency relief co-ordinator Stephen O'Brien warned Monday that new shelter space in tents is currently available for only 60,000 people in camps and emergency sites. Construction of additional sites is under way, but many who flee will take refuge in abandoned buildings such as schools and mosques in nearby villages. There, they may be mistaken as enemy fighters by advancing Iraqi forces.
As well, Iraqi forces will certainly round up all escaping men and boys over the age of 14 in order to screen them for possible security risks, a task that may take weeks and put the displaced men at risk.
Shia Iraqi fighters threaten local Sunnis
The Iraqi Security Forces (ISF), which include the country's armed forces and national police forces, do not have the capacity to capture and secure Mosul and its environs on their own. Their more disciplined Special Forces are reported to be spearheading the attack with the support of Sunni tribal militias and with Iranian-backed Shia militias bringing up the rear. (Kurdish peshmerga forces are carrying out a separate operation further north.)
Iraqi officials insist the Shia militias will not enter the city for fear they may attack the heavily Sunni population. These militias are to remain as a holding force in recaptured areas, as has been the practice in other Sunni cities.
It is apparent, however, that the Shia forces expect to see action. Two of the militias with a record of sectarian violence have pulled 2,000 of their fighters from Syria and deployed them to the outskirts of Mosul.
If nothing more, these Shia fighters may take charge of the security checks on the men and boys who flee Mosul. This has been the case elsewhere and may pose the greatest threat of all to Sunnis.
Kurdish forces have their own agenda
The Kurds of northern Iraq have long claimed parts of Mosul as their own, citing the late Iraqi dictator Saddam Hussein as the culprit who drove Kurds out of the city and "Arabized" the former Kurdish communities.
The Kurds, who hold the majority of terrain surrounding Mosul, also say they will not enter the city centre. Despite that assurance, the Washington-based Institute for the Study of War cautioned Monday that the Kurds "will likely exploit the opportunity to take control of areas by displacing Sunni Arabs from their homes, as they did following operations in nearby Sinjar in November, 2015."
The Turkish presence complicates things
It has no official role in the battle for Mosul, but Turkey's hard-nosed President Recep Tayyip Erdogan insists his country reserves the right to intervene if the Mosul operation fails to go its way. That way, he said earlier this month, is to allow "only Sunni Arabs, Turkmen and Sunni Kurds" to remain in Mosul – that is, no Shiites, no Yazidis and no Christians.
Already, there are Turkish forces inside Iraq, in Bashiqa, close to Mosul. They include hundreds of troops and about 25 tanks, deployed in December, without Baghdad's permission, to safeguard Turkish personnel training Kurds and Arab fighters. Already, the Turkish tanks have shelled IS positions in Mosul.
Iraqi Prime Minister Haider al-Abadi directed the Turkish leader to withdraw his forces from Iraq. Mr. Erdogan, who believes Mosul, once part of the Ottoman Empire, still belongs to Turkey, said the Turkish army does not take orders from Mr. al-Abadi. "I have Turkmen brothers there," he said. "Excuse me, but I won't leave."
Iraqi Shia militias, backed by Iran, have vowed they will turn their guns on Turkish forces if Mr. Erdogan continues his intervention.