Thousands fled Mosul, Iraq, after Sunni extremists took control of the city. Photographer: Emrah Yorulmaz/Anadolu Agency/Getty Images
Thousands fled Mosul, Iraq, after Sunni extremists took control of the city. Photographer: Emrah Yorulmaz/Anadolu Agency/Getty Images

The U.S. has spent almost a decade trying to extricate itself from Iraq. Today's seizure of one of Iraq's most important cities by Sunni extremists may well require the U.S. to become more involved.

A lot rides on the definition of "more involved," of course. U.S. ground troops are out of the question. At the same time, it has been clear for some time that the withdrawal of troops doesn't absolve the U.S. of all its responsibilities to Iraq -- nor did it magically disappear the U.S.'s interests there. In the context of Mosul, what does this mean?

The growing power of the Islamic State of Iraq and the Levant represents a real threat to U.S. interests and stability in the region. ISIL, as it is known, has been expanding since the start of the year, taking Fallujah last month. Now it is in full control of Mosul, a city of 1.7 million people less than 90 miles from the Syrian and Turkish borders. It is a stunning victory for militants who are now fighting in two neighboring countries -- Syria and Iraq -- to create a caliphate that would be run by one of the world's most violent and repressive groups.

So this is not just a local Iraqi problem, pitting an authoritarian Shiite-led government in Baghdad against Sunni radicals. Kurdish militias are stationed just outside Mosul and have said they are ready to drive ISIL from the city. Part of Turkey's 1-million-barrel-per-day Kirkuk-Ceyan oil pipeline will now fall under ISIL's control. Meanwhile, ISIL's Syrian branch, ISIS, has helped attract thousands of foreign jihadis to the fight.

In short, there is a real possibility that the takeover of Mosul could lead to the disintegration of Iraq, with transnational badlands under ISIL's control that serve as a base for the training and radicalization of foreign volunteers.

Much of what is happening in Iraq now is the fault of Iraqi Prime Minister Nouri al-Maliki, who has ruled Iraq in far too sectarian a fashion, alienating the very Sunni leaders who helped to subdue ISIL's precursor in 2007. That complicates matters, and any deal to rescue him should include a binding commitment from him to bring Sunni leaders into the government: These leaders trust the U.S. more than any other player in Iraq.

U.S. involvement would also be needed to overcome the deep tensions and rivalries between the government in Baghdad on one side and the Kurds and Turks on the other. Crushing ISIL may be the one clear common interest they have, yet cooperation is unlikely without diplomatic grease from an outsider, and the U.S. is the only realistic candidate. Even Iran is unlikely to interfere, given its hostility to ISIL (unlike al-Qaeda, ISIL has no tacit nonaggression pact with Iran).

But U.S. involvement would also have to be more than diplomatic. One thing Iraq lacks is a usable air force -- and the surveillance needed to make air support effective. The U.S. can provide such from its airbase just across the Turkish border. It already has allies on the ground, notably the Kurdish Peshmerga, who at this point are far more capable than the U.S.-trained Iraqi military.

The American public is justifiably suspicious of greater involvement in Iraq. It didn't work last time; the very presence of ISIL now is proof. What happens if a U.S. plane is shot down? If Iraqi ground troops prove incapable to the task? If ISIL's strength grows unchecked?

Valid questions all. For now, it is enough to note that the U.S. has already agreed to provide Iraq with missiles and helicopters, and U.S. personnel and equipment are being used to fight terrorists and insurgents in places such as Nigeria and Somalia. U.S. interests in Iraq are worth at least as much of a commitment.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.