With the loss of large parts of Mosul, the second-largest Iraqi city, to Sunni extremist fighters on Tuesday, it is time to recognize that Iraq is once again in a state of civil war. A low-level civil war at present, perhaps, but the situation is only going to worsen unless something dramatic and unexpected pulls Iraq out of its descent. This is a tragedy for Iraq and a serious threat for the United States.
Americans seem to think that the vast increase in domestic oil production from shale deposits has immunized the U.S. economy from Middle East instability. Not by a long shot. The International Energy Agency has warned as clearly as it can that projected low prices of oil in the future depend more on increased Iraqi oil production than on North American shale. And every postwar American recession has been preceded by an increase in oil prices, often the result of Middle East instability.

Then there is the intelligence community's warning that Sunni terrorists waging the Syrian-Iraqi civil war have begun to contemplate striking American targets from the groups' secure base areas in eastern Syria and western Iraq.
Unfortunately, since the height of Iraq's political-military fortunes in 2009-10, the U.S. has squandered and surrendered most of its influence. But we have to hope that there is still time to deal with the mess of Iraq, because the alternative is almost certain disaster.
The key fact to keep in mind is that Prime Minister Nouri al-Maliki is getting desperate for American military aid to regain control of the country. That could give the U.S. the leverage it needs to put Iraqi politics on a firmer footing.
Washington should provide the military support that Mr. Maliki desires—drone strikes, weapons, reconnaissance assets, targeting assistance, improved and expanded training for his forces, even manned airstrikes. But only if he and Iraq's leading politicians agree to settle the deep sectarian conflicts that have brought the country to its present plight.
Iraq's growing crisis is not due to the civil war in Syria or the infiltration of terrorist fighters from abroad. It is rather the fear that Kurds, Sunnis and Shiites have of each other. Each community feels that the others seek to oppress, if not massacre, it and will do so if given half a chance. They also fear a central government with unrestrained power, controlled by one of those communities—in other words, what the Maliki government has become.
Mr. Maliki has so successfully consolidated power—and used it to arbitrarily oust his political and sectarian rivals—that Iraq's other communities, particularly the Sunni Arabs and the Kurds, are terrified of him and of the Shiite Arabs he leads. That is why the Kurds are increasingly pushing toward independence. That is why a Sunni populace that triumphantly evicted Salafi terrorists in 2007-08 is now grudgingly accepting them back. As long as that is the case, Iraq will be riven by civil war and terrorism.
So, in return for a big military aid package, the U.S. should insist on the following:
• A constitutional amendment imposing a two-term limit on the presidency and prime ministership. (A third term for Mr. Maliki may have to be grandfathered in to get him to agree, but simply advertising to all Iraqis that he will not rule for life would be an important reassurance that Iraq is not drifting back into dictatorship.)
• A law defining the powers and prerogatives of the defense and interior ministers, thereby limiting the ability of the prime minister to exercise those powers.
• A law bringing the regional military commands under the ministry of defense chain of command, and thus preventing the prime minister from issuing orders to them directly.
• Passage of a provincial-powers law that delegates significant powers of appointment and greater control of local security forces to the provincial governments.
• A new national-unity government, including a leading Kurd as defense minister and a leading Sunni from one of the opposition parties as interior minister. A national-unity government will be unwieldy and ineffective, but right now it is probably necessary to persuade Iraq's Sunni Arab and Kurdish communities not to abandon Iraq's political process.
• Allowing provinces to attempt to become federal regions, as specified in the Iraqi Constitution.
• The biggest reach of all, but also the best thing for Iraq: a constitutional amendment that redefines Iraq's executive authority, with security and foreign affairs under the president, and the economy and domestic politics under the prime minister.
Collectively, these measures could help undermine Sunni support for al Qaeda-affiliated terrorists who call themselves the Islamic State in Iraq and the Levant. But it is equally important to try to revive Iraq's failing security services.
The politicization of the Iraqi military has been a disaster for the country. Mr. Maliki has worked assiduously to bring the Iraqi security forces under his control by removing many of the competent, professional officers trained, groomed and promoted by the U.S. military. He replaced them with people loyal to himself. The security forces perform poorly now, and they stir fear in the country's minority communities.
The security forces must be depoliticized so that they can play an evenhanded, stabilizing role, as they did in 2008-10. To that end, Washington should insist that Iraq accept thousands of American advisers with full legal immunity and a chain of command running back to Washington. These advisers would need to restore Iraq's disbanded training programs and accompany Iraqi formations in the field to provide firsthand guidance. And, of greatest importance, the U.S. advisers would have to be empowered to promote and remove Iraqi personnel to rebuild its officer corps.
Will any of this happen? Doubtful—the Obama administration seems to turn a blind eye toward Iraq no matter how bad things get. And Baghdad may refuse to surrender its iron grip over the political system. Yet at this late date, if Iraq is to be saved, it will be saved only by Americans and Iraqis taking hard steps like these.
Mr. Pollack is a senior fellow at the Brookings Institution.