By Jessica Lewis

The Wall Street Journal

We’re losing Iraq. Mosul, a great city in northern Iraq, now belongs to the Islamic State of Iraq and al-Sham (ISIS). The world has changed overnight as a former al Qaeda affiliate wrested a major city from a state by force—and without a fight. Within two days of taking Mosul, ISIS pushed south toward Baghdad, collapsing the Iraqi security forces like dominoes in cities from Mosul to Tikrit. ISIS also attacked the Shiite holy city of Samarra. Baghdad is within their sights.

It is already clear that, regardless of whether Baghdad falls, the ascendancy of ISIS is going to redraw and redefine the Middle East. The Kurdish Regional Government, seeing the rest of Iraq in turmoil and government troops pulling out of Kirkuk, moved on Wednesday to secure the oil-rich province—a prize the Kurds had long sought. With the city now controlled by the Kurds’ Peshmerga military, conditions are ripe for the Kurdish region to secede from Iraq. If that happens, the effects are likely to cascade across neighboring states, erasing Middle East borders established in 1916 by the Sykes-Picot agreement. Nevertheless, the greater threat to Middle East sovereign states is ISIS itself, which seeks to establish a transnational emirate. Yet the government in Baghdad would still need the Kurds even if they splinter off: With the Iraqi army in retreat, Iraq may have to enlist the Peshmerga to try to retake the north from ISIS.

These jihadists present a formidable military foe. Disavowed by al Qaeda for unsanctioned advances into Syria, ISIS is a vicious religious organization that possesses daunting military power. Unlike al Qaeda, ISIS has designated its own Islamic state and is expanding it through force. Seated in the city of Raqqa in northern Syria, ISIS has just gained a second regional capital in Mosul and a third in Tikrit, making their cross-border state a reality. The ISIS military campaign in northern Iraq is already a mighty victory—well-designed, well-prepared and well-executed. The question now is how far ISIS can go.

The extremists are encircling Baghdad and likely planning an offensive. But ISIS may move again to strike Samarra, 70 miles to the north and close to the ISIS front line. If these Islamists, who are Sunnis, seize Samarra’s al-Askari mosque—a revered Shiite monument—the country will be thrown into another sectarian civil war. That has long been ISIS’s aim. In a civil war, ISIS thinks it can emerge as the stronger military power. Then the group would have a state, would be fully armed and ready to expand westward, into Syria’s northern cities beyond ISIS-held Raqqa.

The Shiite-dominated Iraqi troops would likely fight to protect Samarra and Baghdad. But the Iraqi military is not at full strength, and its forces are not combat-ready. Desertion, low morale and maintenance deficiencies are rampant. Over the past year, ISIS has thoroughly intimidated Iraqi troops in the north. Three of the four northern army divisions are defunct. The remainder are gathering now in Samarra and Taji, regrouping under other formations for the protection of Samarra and Baghdad.

Another problem: Many of Iraq’s deployable units are already reinforcing Anbar province in the west against ISIS. According to 2013 estimates, the Iraqi army contains 14 maneuver divisions, roughly 200,000 soldiers in addition to 40,000 federal police and 300,000 local police. Four of the army’s divisions are assigned to northern areas of Iraq that have just fallen out of state control. If Iraqi security forces try to retake the north, ISIS would be joined by Baathist elements loyal to the memory of Saddam Hussein and additional insurgent groups in trying to repel them. The Iraqi military simply may not have the capacity to launch a sufficient counteroffensive.

Estimates of ISIS’s full combat strength vary widely, but it is reasonable to equate the jihadists’ ground-maneuver formations in northern Iraq to the military equivalent of roughly 500 soldiers each. The assault force in Mosul reportedly involved between 500 and 800 fighters, traveling in 150 vehicles. ISIS convoys in Mosul and across Iraq have mounted crew-served weapons, including PKM machine guns, and small arms and explosives. Leave-behind forces or additional assault forces required to execute the subsequent attacks likely triple this figure, just in northern Iraq. In all, ISIS fighters in Iraq total at least 4,000. The ISIS equipment laydown is equal to what the Iraqi army can use to fight in an urban environment.

One full Iraqi army division would likely outnumber ISIS in northern Iraq 5-to-1. Yet such superiority in numbers didn’t mean much when ISIS moved on Mosul. And it’s doubtful that Iraq will have such a division available any time soon. For its part, ISIS is moving like a conventional army fighting an expeditionary war, heading toward Baghdad along three parallel lines, dividing the country and moving very fast.

The government of Iraq will likely do everything in its power to protect Baghdad. Large portions of the population, both Shiites and Sunnis who fear the extremists, will also mobilize. But ISIS is a sophisticated, organized and professional force. More than a motivating fear will be needed to repel ISIS and drive it from its newly acquired urban sanctuaries in the north, and Iraq doesn’t appear to have either the army or the military leadership for the job.

There are no political solutions available to Prime Minister Nouri al-Maliki —ISIS doesn’t engage in peace talks. What is needed is a coherent military strategy to halt the present ISIS offensive, and a concerted effort to rebuild Iraqi security forces so that they are armed and trained well enough to oust ISIS from territory it now controls.

In other words, Iraq needs the United States. U.S. Special Operations forces would provide invaluable early-targeting support to Iraqi army units preparing for battle. Airstrikes on ISIS strongholds between Mosul and Bayji would help Iraqi ground forces maneuvering to retake Mosul and Tikrit. The U.S. Army could also provide logistics and other support to the Iraqi military. The Iraqi forces will require additional training, maintenance assistance and battlefield planning support before launching a full counteroffensive. The U.S. can provide it. Drone strikes and other measures suited for combating a terrorist group won’t suffice against ISIS. This is a terrorist army, bent on having its own country.

Ms. Lewis, the research director at the Institute for the Study of War in Washington, D.C., is an Army veteran with service in Iraq and Afghanistan.