As 60 world leaders descend upon Chicago for the NATO summit, the future of NATO's mission in Afghanistan will be center stage -- but NATO's members also will be confronted with a bigger issue of whether the organization can remain relevant.

The challenges going forward are much different and far more complicated than the ones that faced the founders of the North Atlantic Treaty Organization 60 years ago. Then, the premise was simple: an attack in Europe or North America against any member is an attack against all. The Soviet Union was the common enemy that created a shared sense of purpose among NATO allies.

Today, NATO's battlefield knows few bounds, with threats ranging from terrorism and nuclear proliferation to cyberattacks, piracy and disruption of energy supplies.

Since the collapse of the Soviet Union, the alliance has operated mostly away from Europe's backyard.

Missions in Afghanistan and over Libya have reflected a shift from a European defense organization to a global security organization that confronts threats before they reach NATO's border. NATO has also undertaken counterterrorism operations in the Mediterranean Sea, supported African Union peacekeeping missions, conducted anti-piracy operations near the Horn of Africa and assisted with humanitarian relief in Pakistan.

This new global engagement has not always been popular with member-state citizenry, many of whom who saw the missions in Afghanistan and Libya as wars of choice rather than of necessity, ones that cost nations in blood and treasure and put unwanted strains on national budgets during a global financial crisis.

The decade-long NATO involvement in Afghanistan has produced what some have described as collective fatigue among European allies and questions about whether NATO should be involved in conflicts outside of Europe's backyard.

Europe's collective fatigue with NATO's globetrotting has often left the United States shouldering most of the burden, which is considered one of NATO's greatest shortcomings. The United States now covers 75% of NATO defense budgets, while the majority of allies don't even allocate NATO's benchmark 2% of gross domestic product to defense.

Sharp reductions in European defense budgets have only increased dependence on the United States.

Former U.S. Defense Secretary Robert Gates left office with a parting shot to NATO by saying that its unequal burden sharing could erode the alliance. Bemoaning Europe's lack of political will and slashing of its defense budgets, Gates said Congress was tired of footing the bill and warned of a "dim if not dismal future" for the trans-Atlantic alliance if Europe does not step up.

In Afghanistan, several NATO members did not permit their soldiers to fight, focusing instead on development work in quieter areas of the country. And in the recent operation in Libya, only eight members of the alliance took part. The U.S. decision to "lead from behind" in Libya forced members to use their military assets. Yet American aircraft and surveillance were still needed to sustain several of the missions, exposing troubling shortcomings in Europe's defense capabilities and reliance on US military support.

Anders Fogh Rasmussen, NATO's secretary general, has challenged members to embark on "smart defense," essentially pooling assets with the goal of developing and sharing better key military capabilities. While the United States seems ready to meet the challenge, the commitment of NATO's European members, embroiled in the euro crisis, is unclear. 

Although NATO was founded with a pledge to unite a Europe whole and free, plans for enlargement have not been a priority. In a time of austerity, the alliance has sought to reshape itself by strengthening partnership with other states. NATO currently is engaged in partnerships with more than 50 countries in various regional configurations.

Efforts to work closer with Russia, however, have faltered. NATO and the Kremlin agreed to seek ways to cooperate on missile defense but have failed to reach a deal. Moscow wants a legally binding treaty that the missile defense system will not be used to counter Russia's deterrent, but Washington says while the system is not aimed at Russia, it cannot agree to any formal limits on the system.

NATO's reputation for years to come will rise and fall on the future stability of Afghanistan.

The alliance pledged to withdraw its combat forces by 2014. Before then it must train Afghan security forces to maintaining stability in that country. But even after 2014, the alliance's role in Afghanistan will not end and NATO has ensured ensure continued international support even after its troops leave.

Still, once the mission in Afghanistan is over, NATO will be without a major operation for the first time in 20 years.

Some argue that NATO should get back to its basic mission of protecting the homeland security of its allies.

"Maybe we need to work closer to home and focus on threats to security in the trans-Atlantic arena in a more diverse way -- cyber, energy, terrorism and infrastructure protection," says Kurt Volker, a former U.S. ambassador to NATO and now a fellow at the John Hopkins School of Advanced International Studies. "That might generate more public support than sending more troops to a place like Afghanistan."

In an Atlantic Council report on NATO entitled "Anchoring the Alliance," former U.S. ambassador to NATO Nicholas Burns argues that even that even though Europe must show more commitment to NATO's joint security, the United States can't "hand off" responsibilities to NATO.