NATO leaders began a two-day summit here Sunday that will finalize plans to turn control of Afghanistan over to its own security forces by the middle of next year, a milestone on the way to concluding the alliance’s combat role by the end of 2014.

The alliance is meeting at a precarious time for the war effort in Afghanistan, and President Obama and other NATO leaders acknowledged the challenges ahead even as they set out the process for ending the international combat role. NATO members are sharply cutting defense budgets, facing high public opposition to the war and preparing for months of difficult fighting against the Taliban as the number of U.S. and European troops steadily declines across Afghanistan.

As Chicago police confronted demonstrators whose protest against the war and economic policy was kept well away from the heavily guarded summit, Obama welcomed the 27 other government heads to his “home town” and pledged that the 2014 deadline will mean that “the Afghan war as we understand it is over.”

The summit’s initial session dealt with non-Afghanistan issues. The alliance agreed to “operationalize” a missile defense system whose component parts are already in place in four European countries. The system, to protect Europe against ballistic missiles potentially launched from Iran and elsewhere, is expected to have limited capability by 2015 and to be fully operational by 2018.

NATO also signed contracts for its own ground surveillance system, agreeing to purchase and deploy five unarmed Global Hawk drones that will give the alliance capabilities that until now have been available only from the U.S. military. The alliance also moved forward on efforts to more equitably share the cost and contributions to defense operations, now shouldered largely by the United States, and avoid expensive overlaps in capabilities.

No leader raised the subject of the ongoing violence in Syria, said Ivo H. Daalder, U.S. ambassador to NATO.

While “we are very much concerned about the situation of Syria,” NATO Secretary General Anders Fogh Rasmussen said, the alliance has “no intention whatsoever to intervene.”

Although formal meetings on Afghanistan were not scheduled until Monday, that issue is clearly the summit focus. While they prepared to declare that Afghan forces will have the lead military role throughout the country by mid-2013 and looked ahead to complete combat withdrawal at the end of the following year, leaders warned of what Obama called “hard days ahead” between now and then.

“I don’t want to understate the challenge that we have ahead of us,” said Marine Corps Gen. John Allen, the commander of U.S. and coalition forces in Afghanistan. “The Taliban is a resilient and capable opponent,” and “we fully expect that combat is going to continue” as troops are gradually withdrawn over the next 21 / 2 years.

Even as the the mission evolves to an advisory role, Allen said, NATO will retain “short-term capabilities” to shift back into the fight if necessary, even in those regions that have already been “transitioned” to the Afghans. Allen, who spoke to reporters, will give an update Monday to an expanded summit meeting that will include all 62 member nations of the coalition in Afghanistan, plus Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari.

Obama, Karzai meet

Obama held a separate meeting with Karzai early Sunday to discuss the terms of the plan NATO is expected to approve. In public remarks after the 75-minute closed-door session, they mutually acknowledged the sacrifices both nations have made, with Karzai — whose relations with the White House have often been testy — expressing gratitude for “the support that your taxpayers’ money has provided us over the past decade and for the difference that it has made to the well-being of the Afghan people.”

Administration officials said Obama also discussed with Karzai the administration’s hope that the Afghan president, whose term ends in 2014, will move forward on electoral reforms toward a smooth political transition to coincide with the final combat troop withdrawal.

Zardari received a last-minute invitation to the summit after U.S. and Pakistani negotiators indicated progress toward an agreement that would reopen border crossings through which NATO supplies reach Afghanistan. Hopes that the deal would be concluded before the summit were not realized, however, as the two sides continue to haggle over new tariff rates Pakistan wants to impose.

Zardari had no separate meeting with Obama but held a lengthy session with Secretary of State Hillary Rodham Clinton.

NATO’s 2014 exit plan was agreed to at its last summit, 18 months ago in Lisbon. Since then, public antiwar feeling has risen across the alliance, with some members — most recently France’s new socialist government — indicating plans to pull combat troops as early as this year.

A majority of the American public has also turned against the war, and Obama has made his plans to leave Afghanistan a part of his campaign message as he seeks reelection.

The presumptive Republican nominee, Mitt Romney, has accused Obama of aligning his Afghan withdrawal strategy to the political calendar, with 23,000 troops — those remaining from a “surge” deployment of 33,000 Obama authorized in late 2009 — scheduled to come home by the end of September.

The summit is only the third in NATO’s 63-year history to take place in the United States, and it is unfolding against the backdrop of widespread economic crisis and budget cutting among the alliance’s 28 members.

In an opinion piece published in the Chicago Tribune on the eve of the meeting, Romney sharply criticized Obama for the $487 billion the president has proposed in defense cuts over the next decade. Those cuts could double at the end of this year if Obama and Congress do not find a way to cut an additional $1.2 trillion in overall government spending over that period.

Rasmussen, in remarks opening the two-day summit, said the meeting’s focus would be on “security in an age of austerity.”

A Chicago welcome

A few hours after his meeting with Karzai, Obama opened the summit by welcoming his fellow leaders to Chicago, a place where “so many people . . . trace their roots back to NATO countries.”

He called for all member countries to invest in the “defense capabilities and new technologies that meet our collective needs.” NATO can “work together and pool our resources,” he said, allowing “each of our nations to accomplish what none of us can achieve alone.”

One of the issues Obama faces here is the changing role of France in Afghanistan after recent elections that brought Socialist Party candidate Francois Hollande to the presidency. Hollande defeated incumbent Nicolas Sarkozy partly on a pledge to withdraw all 3,300 French troops from Afghanistan by the end of this year, a more rapid withdrawal than had been planned.

“We will need to understand exactly what the French decision will mean,” Allen told reporters, adding that “we will offer France the opportunity to contribute if they wish to the security mission.”

Allen said that could mean that France would increase its participation in “training, mentoring, instructing” Afghan security forces. “All of those,” he said, “are important to the mission overall.” He noted that the U.S. role, too, is changing in much the same way.

“The nature of the mission, as it is evolving now, as our numbers get smaller, is evolving into an advisory mission,” Allen said.

At the Monday meetings, leaders will also discuss the expense of continued support for Afghanistan’s security forces after 2014.

The United States spent $12 billion last year, 95 percent of the total cost, to train and equip an Afghan army and police force that is expected to total 352,000 by this fall. With a gross domestic product of about $17 billion, Afghanistan is incapable of funding a force that size.

As it looks for a way to cut future costs and assumes an eventual political solution to the war among the Afghans themselves, the administration has projected that Afghanistan’s security needs could be met even if the force were cut by up to one-third. It estimates the cost of sustaining the reduced force at about $4.1 billion a year, half of which the United States would provide. Afghanistan would pay about $500,000.

Coalition members have been asked for commitments to fund most of the rest — in annual installments between 2015 and 2017 — up to about $1.3 billion a year. At the summit, “we would like to be able to say that we have a plan, and sufficient resources,” to carry it out, a senior administration official said.

NATO also plans to begin discussion of what the U.S. and NATO presence in Afghanistan will look like after the 2014 withdrawal deadline. The current NATO mission, known as ISAF, for International Security Assistance Force, officially ends at that point, and a new one will have to be agreed upon to cover ongoing training and assistance operations, which are planned to include an estimated tens of thousands of U.S. troops as well as other alliance contributions.