Images emerging from Mosul, Iraq's embattled northern city, present a familiar scene to fans of zombie movies. Burned-out military vehicles are clustered together on empty streets. At every intersection there is evidence of desperate last stands. Strewn uniforms lay abandoned outside gutted police stations. Hundreds of thousands of civilians have fled for their lives. Mosul is a ghost town where only looters and stray dogs hazard the streets.

But this isn't a zombie movie. It is Iraq's second-largest city, the thriving political and economic capital of the country's Sunni Arab community. In a matter of days between June 6 and 9, the city of 1.8 million people was overrun by the Islamic State of Iraq and al-Sham (ISIS), the al Qaeda affiliate that broke away in April 2013 to fight its own war and which has come perilously close to achieving its dream of a caliphate that reaches from the Mediterranean coast of Lebanon to Iran's Zagros Mountains. The scenes from Mosul are now being replayed in a dozen other northern cities that have fallen to ISIS and other insurgent elements.

How did ISIS achieve this coup? Working from a secure base in Syria's Raqqa province, ISIS seems to have carefully prepared operations in Mosul and a range of other cities as the opener to this year's Ramadan offensive — a twisted annual tradition in Iraq since 2003, timed to coincide with the Islamic holy month that spans the length of July this year. The well-publicized ISIS takeover of Fallujah in January 2014 was opportunism, with the movement exploiting missteps by the Iraqi government to move in. In stark contrast, the Mosul assault appears calculated and deliberate, an attempt to collapse government control in northern Iraq and leave ISIS as the last man standing. So far, ISIS has been successful.

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Formerly known as al Qaeda in Iraq, ISIS is an Iraqi-led militant group that draws its suicide operators primarily from international volunteers, its foot soldiers from Iraq and Syria, and its money chiefly from a mix of local organized crime rackets. ISIS has used the Syrian conflict to build its strength and assert its independence from al Qaeda's senior leadership in Pakistan, which disavowed it in February 2014. But the movement's real ambitions rest in Iraq. The majority of ISIS's leadership and rank and file are Iraqi, including its emir, Abu Bakr al-Baghdadi. Control of western Mosul would place ISIS in charge of the political and economic capital of Sunni Iraq, a prize of tremendous propaganda value.

ISIS used battle-hardened fighters from the Syrian and Iraqi theaters to smash their way — in a matter of hours — into Mosul's western neighborhoods from the Jazira, the Syrian-Iraqi desert between the Tigris and Euphrates rivers. The Iraqi government's weak grasp on the Jazira gave the movement the capacity to surge hundreds of fighters from Iraq and Syria into the battle of Mosul. Even so, the ISIS attack force does not appear to have been large, numbering between 400 and 800 fighters by various estimates. Surprise and aggression allowed the movement to crumble the morale of Iraq's paramilitary police and army forces in Mosul in three days of hard fighting.

ISIS took on and defeated a government force 15 times their size.

Let that sink in.

The 20 government security battalions in Mosul city seemed to dissolve completely on June 8-9, with significant video evidence of wholesale abandonment of positions by troops who ditched their vehicles and posts, took off their uniforms, and deserted. One photo shows a discarded police brigadier-general's uniform. The two main security headquarters in Mosul were both overrun and looted by ISIS, as were the provincial governor's offices. The Mosul branch of Iraq's Central Bank has reportedly been looted and the historic Assyrian church set aflame. The Iraqi army logistics depot was abandoned to ISIS, who are reported to have burned over 200 U.S.-provided Hummers, trucks, and engineering vehicles. Mosul's international airport and military airfield also fell to ISIS, with Iraqi helicopters reported destroyed on the ground and armored vehicles being quickly taken to Syria as war booty.

A desperate race has now commenced for control of Mosul. ISIS has achieved its basic aim of shattering any rival political or military institutions on the predominately Arab side of Mosul west of the Tigris River. Now it will try to rapidly reinforce the city using its open road across the desert to Syria and other Iraqi districts connected via the western desert. With the government's control shaky all the way to Baghdad's northern suburbs, ISIS has a shot at causing so much disruption that Baghdad will not be able muster the forces to retake Mosul for months.

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The Iraqi government has no choice but to try to liberate Mosul, due to its symbolic and geographic importance. Atheel al-Nujaifi, the Mosul-based governor of Nineveh province, took to the streets on June 9 when he narrowly escaped the ISIS takeover of his offices. Carrying an assault rifle alongside his security detail, he sought to rally Mosul men to form self-defense militias at the neighborhood level. But such militias do not seem to have mobilized. All across northern Iraq, the Sunni citizenry will need to see the government return in force before they risk being pounded by ISIS car bombs, which would be the movement's first recourse if local resistance were to emerge.

With shattered Iraqi military units rallying as far away as Taji, a base on Baghdad's suburbs some 200 miles south of Mosul, the government's counteroffensive could be slow in coming. Baghdad's soldiers now have to fight their way through a belt of lost cities and districts between the capital and Mosul, creating plenty of potential distractions, which will drain strength away from the government riposte. Special forces and air units are reportedly rapidly becoming exhausted as they are shuffled from crisis to crisis. The only military force in Iraq that is not presently overcommitted is the pesh merga, the Kurdish fighters controlled by the Kurdistan Regional Government, but relations between Baghdad and the Kurdish region are particularly strained.

Seeking the overthrow of Bashar al-Assad, the United States has been forced to walk a fine line with jihadist groups in Syria. ISIS was only confirmed as a U.S.-designated terrorist movement in February 2014. But while there may be a strategic use for hard-line Islamist militants in Syria, in Iraq the issue is simple: ISIS is winning the war and they must be stopped.

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Washington must act if the United States wants to stop ISIS from becoming the only cohesive military and political force in Iraq's Sunni districts. On June 10, Osama al-Nujaifi, Iraq's parliamentary speaker and most senior Sunni politician, requested greater military support for Mosul under the auspices of the 2011 U.S.-Iraq Strategic Framework Agreement, the treaty that governs relations between the two countries. Behind closed doors, multiple Iraqi government officials relayed to me, the Iraqi government has insistently requested U.S. air strikes on ISIS along the Syrian border and the outskirts of Iraqi cities, which are the launch pads for ISIS takeovers. For the U.S. administration this has been seen as a step too far. Instead, the U.S. government has been engaged in internecine diplomacy — using its good offices to prod Iraq's factions towards a national reconciliation effort that could give Sunni Arabs faith in a nonviolent resolution to their complaints of discrimination by the Shiite government. Reconciliation could also lay the groundwork for Sunni Arab cooperation in stabilizing Mosul and other lost areas, such as Fallujah. This is vital work — but with ISIS forces capturing city after city, Washington has to do more (and quickly) to prevent the loss of government in Iraq. Intensified U.S. on-the-ground mentoring of Iraqi military headquarters and perhaps U.S. air strikes might also be needed to reverse the collapse of Iraq's military.

The Obama administration is determined to honor its campaign pledge to end the wars. To that end, the White House withdrew U.S. combat troops in 2011. However there is an increasingly strong case that Iraq needs new and boosted security assistance, including air strikes and a massively boosted security cooperation initiative to rebuild the shattered army and mentor it in combat. The Middle East could see the collapse of state stability in a cross-sectarian, multiethnic country of 35 million people that borders many of the region's most important states and is the world's fastest-growing oil exporter. Any other country with the same importance and the same grievous challenges would get more U.S. support, but the withdrawal pledge has put Iraq in a special category all on its own. Washington doesn't have the luxury of treating Iraq as a special case anymore. ISIS has moved on since the days of the U.S. occupation and they have a plan. Washington should too.