The leaders have crossed a bridge across the Rhine from Germany to France in a symbol of unity ahead of talks that will include frank exchanges over issues ranging from strategy in Afghanistan to relations with Russia and a possible hold on any further expansion.
Split between the towns of Kehl in Germany and Strasbourg in France, the meeting will also see France formally rejoin NATO's military command structures, while Croatia and Albania will be welcomed as the alliance's 27th and 28th member states.
But the summit is first and foremost a stock-taking exercise.
The summit is U.S. President Barack Obama's first appearance at an alliance meeting, and European allies in particular are keen to receive a firsthand briefing of the intentions and priorities of NATO's largest member state.
The summit will launch a debate on the alliance's new strategic concept, in effect NATO's mission statement. The current concept, adopted in 1999 and defining the alliance as "Eurocentric," is widely seen as badly out of date.
The new concept represents an effort to find a universally satisfactory balance between NATO's original core mission of mutual defense and its increasing focus since the 9/11 terrorist attacks on out-of-area missions, chief among them Afghanistan.
U.S. officials have made it clear since Obama's inauguration in January that the stabilization of Afghanistan and Pakistan -- particularly the restive "Pashtun belt" thought to remain an Al-Qaeda sanctuary -- is Washington's top global foreign policy priority.
Washington has announced the deployment of an extra 21,000 troops to the country and asked European allies to follow suit. Significant new European troop contributions to what one German columnist this week called the "unloved war" remain highly unlikely at the summit, however, with European allies likely to display some anxiety -- if well-camouflaged -- over Obama's evident belief in the benefits of military force.
As if to underscore the challenge before the U.S. administration, French President Nicolas Sarkozy said at a joint press conference after Obama's arrival in Strasbourg that "there will be no additional French troops [in Afghanistan] because the decision to step up our troop presence was already made last year."
A NATO source in Strasbourg predicted on April 3 that the alliance would announce that it is taking greater control over police-training efforts in Afghanistan, however. The source told RFE/RL that the French, Italians, and Portuguese are likely to follow that annoucement in the coming days by saying that they will supply some 300 additional trainers to bolster the Afghan military police. The EU currently has 177 conventional police trainers in the country.
Afghan national police, and specifically military police, are increasingly being identified as crucial in helping the Afghan government hold ground won by NATO's ISAF mission and the Afghan National Army, which in turn is essential for reconstruction.
Russia As A Partner
Another major tangle of issues surrounds Russia. A U.S. policy about-face saw NATO restore full diplomatic relations with Moscow last month after a seven-month hiatus brought about by the Russia-Georgia war. Eastern European allies, feeling vulnerable, have put pressure on NATO to devise defense plans against a possible Russian attack.
That is unlikely to happen, however, given Obama's frequently reiterated wish to give a new start to relations with Moscow. After a meeting in London on April 1, Obama and Russian President Dmitry Medvedev issued a statement saying they want to "move beyond Cold War mentalities."
Russia goodwill is crucial for another of Obama's major policy objectives, the struggle against nuclear proliferation. In the coming months, the United States and Russia are expected to begin negotiations on a series of disarmament accords, the most pressing among them a successor to the START treaty set to expire in December.
Russian cooperation could also greatly facilitate curbing Iran's nuclear ambitions, and the country's importance as a transit corridor for NATO supplies bound for Afghanistan is growing as Pakistan succumbs to instability.
The Obama administration has signaled a possible willingness to put the brakes on a missile-defense project in Eastern Europe and indefinitely delay any further NATO enlargement in the vicinity of Russia.
In fact, the welcome that NATO's 26 other leaders will extend to their new colleagues from Albania and Croatia -- which on April 1 became the alliance's 27th and 28th member states -- could mark the end of NATO expansion for the foreseeable future. The only country now in the NATO waiting room, Macedonia, must overcome resistance from Greece, which objects to its name.
The alliance will also need to readjust to readmit France into its military command structures. Creating a number of top-level positions for French generals is likely to be the tip of the iceberg in this process, with Paris intent on setting up a "European pillar" within the alliance.
Sarkozy also appears to be genuinely concerned about developing European defense potential. In February, at a high-level security conference in Munich, he warned that the continent must decide whether what it wants is "peace, or to be left in peace." Its political rhetoric in the recent past notwithstanding, France has actively contributed troops to the ISAF mission in Afghanistan and appears supportive of U.S. attempts to craft a broader, global mission for NATO.
The same cannot be said of Germany, whose Chancellor Angela Merkel said on March 27 that there will be "no global NATO." This is a sentiment shared by Germany's Social Democrat Foreign Minister Frank-Walter Steinmeier, who said on April 1 that NATO must not be a "world policeman." Thus, Germany is set to oppose attempts to give NATO a new lease on life as the guarantor of what Roger Cohen described this week in "The New York Times" as "the defense and expansion of the liberal democratic order."
In a piece which appeared in the daily "Frankfurter Allgemeine Zeitung," the foreign policy spokesman of Merkel's Christian Democrat faction in the German Bundestag, Eckart von Klaeden, argued NATO must develop a "comprehensive approach" to crises going beyond the application of military force.
That suggests closer cooperation with the EU -- which should be easier after the French return into the NATO fold, Eckart noted -- but also a drive by the alliance to hone its own civilian capabilities in places like Afghanistan. Von Klaeden suggested that NATO's interior and development ministers could hold periodic meetings in the manner of their colleagues in the foreign and defense ministries to discuss, respectively, training Afghan police and reconstruction.
Germany -- which contends it is shouldering its share of the burden in Afghanistan in the country's relatively untroubled north -- this week assumed leadership of a "support group" for Afghanistan to coordinate the international reconstruction effort in the country. The group, comprising special envoys from the EU, United Nations and NATO, as well as 12 countries, met in Munich on April 1.
Germany (and France) are likely to welcome Obama's change of policy on Russia. Neither wants to antagonize Russia, which is seen in Europe as a vital, if difficult, strategic partner.
Some of the debate will also focus on the specificities of novel threats such as cyberattacks and disruptions of energy supplies, which some allies argue could in certain circumstances fall within the purview of Article 5, NATO's mutual defense clause -- although, as it stands, Article 5 only authorizes the triggering of the mutual defense commitment in the event of an "armed attack" on one of the allies.
60 Years In Eight Minutes
In Washington in 1949, the North Atlantic Treaty Organization was born. RFE/RL looks at the dramatic and difficult moments in NATO history with rarely seen archive films and exclusive interviews. Play
NATO At 60 series:
Rolling With The Changes
End Of Expansion?
The Article Of Faith
Getting The Balance Right