The Iraqi government has lost control of its third-largest city to Al Qaeda-inspired insurgents, a crushing defeat for not only Prime Minister Nouri al-Maliki's security policies but for Iraqi politics as a whole.

The scale of the catastrophe, as troops loyal to Mr. Maliki flood north and troops controlled by the Kurdish Regional Government rush west and south, can't be overstated. Chicago is the United States' third-largest city. Munich is Germany's. Osaka is Japan's.

And unlike the Anbar towns of Fallujah and Ramadi, almost exclusively Sunni Arab and in the heart of what has long been one of Iraq's most restive provinces, Mosul is an ethnically and religiously mixed town of Sunni and Shiite Arabs, Kurds and Turkmen, Christians and Muslims. US forces won, lost, and won control again of Fallujah in fierce battles during the early years of the America-led war in Iraq. But a city like Mosul is something else again.

It's well known that Mosul has been a target for the Islamic State in Iraq and the Levant (ISIS). The city is the capital of northern Nineveh Province, the western side of which has a roughly 300-mile-long frontier with Syria. During the height of the US war in Iraq, insurgent rat-lines riddled the border, and in the past few years, with what was once Al Qaeda in Iraq merging with Sunni Arab insurgents fighting in the Syrian civil war to become ISIS, the cross-border flow of men and weapons has ramped up again.

Much of Nineveh, like Anbar, is sparsely inhabited desert where the central government's writ is nominal. Smaller cities in the area's east, like Tal Afar, have repeatedly fallen to insurgents over the past decade. But Mosul is a crown jewel, a center of transportation and commerce. Holding it was a government priority. Losing control, if only briefly, is a powerful indication of government failure and something that is likely to spur insurgent recruitment. What must have looked like a hopeless cause to many passive Sunni Arab supporters of the insurgency just started looking a lot more hopeful.

Maliki has responded by declaring a state of emergency, imposing a curfew in Baghdad and other cities, and, in a move that smacks of desperation, issuing calls to arm citizen irregulars to fight the well-organized and armed ISIS military.

Iraqi Parliament Speaker Osman Nujaifi said virtually all government installations in the city have fallen into ISIS hands. "When the battle got tough in the city of Mosul, the troops dropped their weapons and abandoned their posts, making it an easy prey for the terrorists," Mr. Nujaifi told reporters, according to a Los Angeles Times translation. "Having these terrorist groups control a city in the heart of Iraq threatens not only Iraq but the entire region."

Today, according to people in the city, organized ISIS fighters dismantled the city's security barricades and roadblocks. While there were reports that the portion of the city east of the Tigris River, closer to the Kurdish heartland, had not fallen, western Mosul is the heart of the town.

The ISIS victory is a signature moment – evidence that ISIS can't be dismissed as merely a ragtag group of insurgents who may be able to hold their own in Syria, but would be unlikely to take and hold ground from Iraqi forces. The US spent more than $14 billion on training and equipping Iraqi security forces. When US forces left Iraq at the end of 2011 (because Maliki refused to sign an agreement extending US military involvement), US politicians and military leaders spoke of how the Iraqis were ready to stand on their own, how the seeds of political reconciliation had been sown by a war that cost more than $2 trillion, 4,486 American lives, and more than 100,000 Iraqi ones.

"We remember the surge and we remember the Awakening when the abyss of chaos turned toward the promise of reconciliation.... In handing over responsibility to the Iraqis, you preserved the gains of the last four years and made this day possible," President Barack Obama said in a speech to troops shortly before the withdrawal. "In an area that was once the heart of the insurgency, a combination of fighting and training, politics and partnership brought the promise of peace."

This turned out to be an illusion in Anbar Province, in part thanks to Maliki and his Shiite allies' decision to turn on the Iraqi tribes that made up the Sunni "Awakening." It's looking like an illusion also for much of the rest of the country, where politics remain defined along sectarian and ethnic lines, and where the toll of fighting on civilians and government forces alike have returned to the levels of 2007-2008.

This is Mosul today, through the eyes of Agence France-Presse reporters in the city.

An AFP journalist, himself fleeing the city with his family, said shops were closed, a police station had been set ablaze and that numerous security force vehicles had been burned or abandoned. Hundreds of families were seen fleeing. Some were on foot, carrying what they could, others in vehicles with their belongings piled on the roofs. Another AFP journalist said thousands of Mosul residents had fled for the safety of the autonomous Kurdish region in the north. Dozens of cars and trucks stretched out from one checkpoint on the boundary of the region, as people with plastic bags, suitcases and a pram waited to enter, some with young children in tow. "The army forces threw away their weapons and changed their clothes and left their vehicles and left the city," said Mahmud Nuri, a displaced Mosul resident. "We didn't see anyone fire a shot."

Events in the city today are a stark reminder of how ephemeral US efforts in Iraq have proven to be. In early 2004, Gen. David Petraeus was commander of the 101st Airborne Division in the province, and his efforts there, focusing on hearts and minds, were marketed as the "Mosul model." Early in the war, Mosul was Iraq's most peaceful large city, new businesses were opening, and fuel shortages that bedeviled most of the country then weren't apparent.

At the time, the Bush administration, the military, and the US people were still expecting a quick war. That January, Petraeus's 18,000 troops in the region were being replaced by a force of about 5,000.

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Petraeus said then:

"They will have the benefit of a substantially larger Iraqi security presence coming on line,'' says General Petraeus, whose unit has trained more than 10,000 Iraqi soldiers, border guards, and police. "This is an occasion where we'll see how the new Iraqi security forces are going to do. I think they'll be fine." Petraeus has been slowly pulling his forces back since September, seeking to hand over more and more authority to a local governor and council selected shortly after the 101st arrived in the Mosul area last April. "We're only six months away from June and handing control of the country back over to Iraqis," he says. "The number of joint patrols we run on the border with Syria, for instance, has been steadily decreasing as the capabilities of local forces have increased,'' says Maj. Mike Getchell, who serves in the 101st's Third Brigade under Col. Mike Linnington, outside the city of Talafar.

Ten years on, Iraq does not control its border with Syria and it does not control Mosul. If ISIS manages to hang on to the city, even if only for a short while, it will be able to threaten towns farther south and closer to Baghdad, and have greater freedom to organize suicide bombings, something that could spark a major sectarian war like the one that raged in the middle of the past decade. Maliki's call for arming civilians probably means he intends to use Shiite militias in an effort to regain control.