More than 50 world leaders were gathering in Chicago for one of the biggest NATO summits in history Sunday aiming to hammer out a unified exit strategy from Afghanistan after a decade of war.

A huge security operation has swung into place in the political backyard of US President Barack Obama, with police deployed along the main arteries, some on horseback, as Coast Guard boats topped with machine guns patrol the river.

It is the first summit of the 28-member North Atlantic Treaty Organization on US soil in more than a decade, and follows a two-day summit of G8 leaders hosted by Obama in the seclusion of Camp David, Maryland.

In a sign of heightened tensions, authorities in the Windy City, already bracing for massive protests, charged three men Saturday with plotting to attack Obama's campaign headquarters and other targets during the summit.

Despite a myriad of issues facing the 63-year-old organization founded in the wake of World War II as it confronts shifting 21st-century realities, the Chicago summit is set to be dominated by Afghanistan.

According to The New York Times, Obama will announce at the gathering what he has already told the leaders in private: All combat operations led by US forces will cease in the summer of 2013, when the United States and other NATO forces move to a "support role", whether the Afghan military can secure the country or not.

Obama arrived back in his hometown late Saturday, met by cheering crowds who lined the route as his motorcade drove downtown.

Among the world leaders at the table with Obama will be Afghan President Hamid Karzai and his Pakistani counterpart Asif Ali Zardari, who accepted a last-minute invitation to attend.

Despite the stubborn Taliban insurgency, war-weary international forces are seeking to hand control of security to Afghan forces while withdrawing some 130,000 foreign combat troops by the end of 2014.

Karzai comes armed with a firm demand for $4.1 billion (3.2 billion euros) a year to fund his security forces after the pullout -- fearing his country could descend into a new civil war.

The United States is expected to foot half the bill while hoping the international community will stump up the rest.

Washington is also hoping that Zardari will agree to reopen key NATO supply routes into Afghanistan closed in November after US air strikes killed 26 Pakistani troops.

But US Defense Secretary Leon Panetta suggested in an interview with the Los Angeles Times that Islamabad was demanding too high a price to resume the deliveries across its territory.

Quoting a senior US official, the daily said Islamabad now wanted $5,000 per truck, compared with $250 previously, amounting to a goodly sum for the thousands of trucks that trundle across the border daily.

"Considering the financial challenges that we're facing, that's not likely," Panetta told the daily.

Pakistan's cooperation however is seen as key to the success of the international mission in Afghanistan, as US-led NATO forces fight a fierce Taliban insurgency.

"We can't solve the problems in Afghanistan without the positive engagement of Pakistan," NATO Secretary General Anders Fogh Rasmussen said Saturday.

The White House was hoping the issue of the supply routes would soon be resolved, said deputy national security adviser Ben Rhodes on Saturday.

"Based on the statements they've made, the negotiations going on, we believe it's going to be accomplished," he told reporters traveling with Obama.

"We're not anticipating necessarily closing out those negotiations this weekend."

Indeed, planned talks between Rasmussen and Zardari were abruptly called off late Saturday, officially because the Pakistani leader's flight was late arriving from London. Both sides said they hoped to reschedule the meeting.

France's new President Francois Hollande has meanwhile shaken up the carefully crafted transition plan, vowing to bring his 3,500 combat troops home in 2012, a year earlier than planned.

Obama and his fellow leaders will take other key decisions, activating the first part of a missile shield for Europe and announcing a slew of military cooperation projects to cope with mounting austerity.

Governments are feeling the pinch as Europe's debt crisis forces budget cuts across the board, and to cope NATO will announce more than 20 joint projects to pool military hardware as part of a so-called "Smart Defense" initiative.

NATO has touted a planned US-led missile shield for Europe as a shining example of military cooperation.

A first step to the shield will be the announcement at the summit of an "interim capability" putting US warships armed with missile interceptors in the Mediterranean, and a radar system based in Turkey under NATO command.

The top issues at the NATO summit
Chicago (AFP) May 20, 2012 - The war in Afghanistan, a missile shield for Europe and the pooling of military resources in times of austerity top the agenda of the NATO summit in Chicago on Sunday and Monday.

AFGHANISTAN

US President Barack Obama and fellow NATO leaders will fine-tune plans to hand security control to Afghan troops and withdraw 130,000 combat troops by late 2014.

The 28-nation alliance will discuss post-2014 support, which is expected to focus on continuing to provide training to Afghan security forces.

NATO will debate the size of the Afghan force and the funding it will need after 2014. The military and police force will grow to 352,000 this year but a US proposal foresees a reduction to 228,000.

The future cost to support the Afghan troops is estimated at $4.1 billion per year. The United States is expected to foot half the bill and Washington wants the international community to provide the rest.

MISSILE SHIELD

NATO will launch the first phase of a US-led missile shield to protect Europeans from the threat of ballistic missiles from foes such as Iran despite Russian opposition to the project.

The announcement of an "initial capability" will mean that US warships armed with missile interceptors in the Mediterranean and an early-warning radar system in Turkey will come under the command of a NATO base in Ramstein, Germany.

The missile defense system is being deployed over several years. Poland and Romania have agreed to host US land-based SM-3 missiles in coming years while the US Aegis ships are based in a Spanish port.

Scheduled to become fully operational in 2018, the system has irked Russia, which fears it will undermine its nuclear deterrent and has threatened to deploy weapons to EU borders in response.

To ease Russian concerns, NATO has urged Moscow to cooperate in the system but the two sides have struggled to find a compromise.

SMART DEFENSE

The summit will be the launching pad for NATO's "Smart Defense" initiative, a push to encourage joint military projects in order to make up for dwindling budgets at a time of austerity across the alliance.

Between 20 and 25 projects will be announced in Chicago, ranging from training helicopter pilots to the joint management of munitions.

The United States, which accounts for 75 percent of NATO's military spending, has pressed European allies for years to pull their own weight, but the debt crisis is making it even more difficult for them to invest in new weapons.

The NATO-led air war in Libya last year drove home the transatlantic disparity and the stark realities for Europe's armed forces: a serious shortage of aerial refueling tankers, surveillance drones and precision-guided bombs.

PARTNERSHIPS

NATO wants to strengthen its partnerships around the world, drawing from the success of its cooperation with Qatar, the United Arab Emirates and Jordan in the air campaign in Libya last year.

The leaders of the 22 nations participating in the NATO-led mission in Afghanistan were also invited to the summit.

NATO wants to deepen relations with traditional partners such as Australia, Japan, South Korea and New Zealand.

But three military powers not involved in NATO operations, China, India and Israel, were not invited to the summit.

Russian President Vladimir Putin declined to attend.