Mosul's geography is also of significant strategic importance. It is located on the Tigris River, giving it access to water trade routes, and it is also home to pipelines that carry oil into Turkey. The city is also less than 100 miles from Syria, giving the group a potentially strong foothold to control territory on both sides of the border.
"What they're looking to do is erase the border. They are looking to set up a unified state within Iraq and Syria," Pollack says. Furthermore, he adds, Mosul has produced a significant number of the Iraqi military's officer corps as well as influential politicians.
Michael Knights, an expert on Iraqi security at the Washington Institute for Near East Policy, says that ISIL has operated in Mosul "like mafiosi for years, operating in semi-plain sight, controlling all the organized crime rackets, using the city as a fundraising center."
"Now they're going for open control. And that is something that no faction in Iraq can ignore anymore," Knights says.
The sheer size of the target is a demonstration of ISIL's capabilities, Pollack says. "Fallujah, Ramadi: Those are not insignificant-sized towns, but they're not major cities. Mosul is a major metropolis. It would be like a terrorist group attacking Dallas or Philadelphia. These are very big urban areas that they are now trying to take on, and it indicates a degree of strength that I don't think anyone recognized that they had."
Knights says that the ISIL seizure of government buildings in Mosul goes "above and beyond normal chaos in Iraq."
"It goes above and beyond losing Fallujah, a place that everyone's quite happy to write off, typically," Knights adds. "This is a place that nobody can write off. It's a critical economic hub in the country."
The reports of Iraqi security forces fleeing the ISIL attackers demonstrates the crucial role that morale plays in armed conflicts in the country, Knights says, quoting the Napoleon Bonaparte axiom that "the moral is to the physical as three to one."
"In Iraq it's like six to one. So, if you get 400 really determined fighters, they can make a security force of thousands throw down their weapons and uniforms and run away," he explains.
Iraqi Prime Minister Nouri al-Maliki has asked the parliament to declare a state of emergency, and Iraqi media reported that lawmakers would meet on June 12 to consider the appeal. On June 9, the governor of Iraq's northern Nineveh province, Athil al-Nujaifi, delivered a televised address asking Mosul residents to fight the militants.
Meanwhile, the governor's brother, parliament speaker Osama al-Nujaifi, asked Iraq's government and Kurdistan's regional administration to deploy forces to Mosul. He also said that he also reached out to U.S. Ambassador Robert Beecroft to request assistance.
Knights says that it was not unusual for Iraqi security forces to flee an initial assault and then regroup. "They collapse at the start of a fight, they get up, they dust themselves off, and they go back," he adds. "And then, without the element of surprise against them, they do okay. And that's probably what we're going to see next."
This post appears courtesy of Radio Free Europe/Radio Liberty.