Friday afternoon is a traditional time to bury bad news, so at 12:49 p.m. on Oct. 21 President Obama strode into the White House briefing room to “report that, as promised, the rest of our troops in Iraq will come home by the end of the year—after nearly nine years, America’s war in Iraq will be over.” He acted as though this represented a triumph, but it was really a defeat. The U.S. had tried to extend the presence of our troops past Dec. 31. Why did we fail?

The popular explanation is that the Iraqis refused to provide legal immunity for U.S. troops if they are accused of breaking Iraq’s laws. Prime Minister Nouri al-Maliki himself said: “When the Americans asked for immunity, the Iraqi side answered that it was not possible. The discussions over the number of trainers and the place of training stopped. Now that the issue of immunity was decided and that no immunity to be given, the withdrawal has started.”

But Mr. Maliki and other Iraqi political figures expressed exactly the same reservations about immunity in 2008 during the negotiation of the last Status of Forces Agreement. Indeed those concerns were more acute at the time because there were so many more U.S. personnel in Iraq—nearly 150,000, compared with fewer than 50,000 today. So why was it possible for the Bush administration to reach a deal with the Iraqis but not for the Obama administration?

Quite simply it was a matter of will: President Bush really wanted to get a deal done, whereas Mr. Obama did not. Mr. Bush spoke weekly with Mr. Maliki by video teleconference. Mr. Obama had not spoken with Mr. Maliki for months before calling him in late October to announce the end of negotiations. Mr. Obama and his senior aides did not even bother to meet with Iraqi officials at the United Nations General Assembly in September.

The administration didn’t even open talks on renewing the Status of Forces Agreement until this summer, a few months before U.S. troops would have to start shuttering their remaining bases to pull out by Dec. 31. The previous agreement, in 2008, took a year to negotiate.

The recent negotiations were jinxed from the start by the insistence of State Department and Pentagon lawyers that any immunity provisions be ratified by the Iraqi parliament—something that the U.S. hadn’t insisted on in 2008 and that would be almost impossible to get today. In many other countries, including throughout the Arab world, U.S. personnel operate under a Memorandum of Understanding that doesn’t require parliamentary ratification. Why not in Iraq? Mr. Obama could have chosen to override the lawyers’ excessive demands, but he didn’t.

He also undercut his own negotiating team by regularly bragging—in political speeches delivered while talks were ongoing—of his plans to “end” the “war in Iraq.” Even more damaging was his August decision to commit only 3,000 to 5,000 troops to a possible mission in Iraq post-2011. This was far below the number judged necessary by our military commanders. They had asked for nearly 20,000 personnel to carry out counterterrorist operations, support American diplomats, and provide training and support to the Iraqi security forces. That figure was whittled down by Defense Secretary Leon Panetta and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, to 10,000, which they judged to be the absolute minimum needed.

The Iraqis knew about these estimates: U.S. military commanders had communicated them directly to Iraqi leaders. Prime Minister Maliki was said (by those who had talked to him) to privately support such a troop commitment, and almost all Iraqi political leaders—representing every major faction except for the rabidly anti-American Sadrists—assented on Aug. 2 to opening negotiations on that basis.

When the White House then said it would consent to no more than 5,000 troops—a number that may not even have been able to adequately defend itself, much less carry out other missions—the Iraqis understandably figured that the U.S. wasn’t serious about a continued commitment. Iraqi political leaders may have been willing to risk a domestic backlash to support a substantial commitment of 10,000 or more troops. They were not willing to stick their necks out for such a puny force. Hence the breakdown of talks.

There is still a possibility for close U.S.-Iraqi military cooperation under the existing Strategic Framework Agreement. This could authorize joint exercises between the two countries and even the presence of a small U.S. Special Operations contingent in Iraq. But it is no substitute for the kind of robust U.S. military presence that would be needed to bolster Iraq’s nascent democracy and counter interference from Iran, Saudi Arabia and other regional players that don’t have Iraq’s best interests at heart.

Iraq will increasingly find itself on its own, even though its air forces still lack the capability to defend its own airspace and its ground forces cannot carry out large-scale combined arms operations. Multiple terrorist groups also remain active, and almost as many civilians died in Iraq last year as in Afghanistan.

So the end of the U.S. military mission in Iraq is a tragedy, not a triumph—and a self-inflicted one at that.

Mr. Boot is a senior fellow in national security studies at the Council on Foreign Relations.