Some controversies fade so quickly, and seem so silly in retrospect, that people forget they ever took place. But they can be instructive. For example: Less than a decade ago, there was intense public debate over whether NATO should be enlarged with new members from Central and Eastern Europe. The issue was closely linked to the war in Bosnia; the foreign policy establishment in the United States and Europe – including many commentators, both liberal and conservative – largely opposed enlarging NATO, and most senior diplomats and bureaucrats thought it would destabilize Europe.

The majority of old NATO hands, from Paul Nitze to George Kennan, and the Pentagon itself, were also opposed. But, after a fierce debate, President Bill Clinton came to the (accurate) conclusion in 1994-95 that it would enlarge the area of stability in Europe; he then gained the support of key European governments, leading the alliance to a historic consensus that has added, in two phases, 10 new NATO members so far.

Today, that decision looks like a no-brainer; it is hard even to remember how contentious it was. The critics, although few have ever acknowledged it, were wrong. Without NATO’s enlargement, we would face greater instability in Central Europe, especially in the Balkans, as well as greater problems with a semi-irredentist and increasingly authoritarian Russia. NATO would probably have atrophied and become increasingly irrelevant, because it would have failed to address the historic security issues of that decade: stopping ethnic cleansing in the Balkans and helping to create a new Europe that was, for the first time in history, democratic, integrated and free.

NATO now must decide whether to begin what we would call the third phase of its history. Phase I was the alliance’s creation in the late 1940s and the Cold War; Phase II was the enlarging and reorganizing of NATO, post-Cold War, and dealing with Bosnia and Kosovo. Phase III poses a fundamental question: Should NATO play a role in dealing with problems outside its historical “space”? (It is relevant to recall that until 1995 most NATO wonks said that Bosnia lay outside that space, even though NATO countries virtually surrounded the Balkans.)

The gravest threats to its members’ collective security now come thousands of miles from the European heartland, not just a few yards away on the East Berlin side of Checkpoint Charlie. If NATO does not take on more of these problems, we will all be less safe, and the alliance will again risk becoming irrelevant.

This doesn’t mean NATO should become a globo-cop; not every security problem in the world is of direct concern to NATO, and not every issue can be solved by it. Nor is this a call for a new age of Western imperialism; we are talking here of dealing with issues of national security vital to all NATO members – issues that happen to lie outside NATO’s traditional area of concern but on which NATO can make a difference. In fact, NATO has put its toe in some global waters by (belatedly) taking over a major mission in Afghanistan authorized by the United Nations, starting a modest training mission for Iraq, flying relief missions to the earthquake zone in Pakistan, and beginning (again, belatedly) to discuss a significant, U.N.-authorized role in Darfur.

These are commendable actions, but NATO has not yet crossed the Rubicon and explicitly embraced a more global mission. Each individual NATO action thus becomes the arena for an internal battle royal. It is time for a formal policy decision to be made soon, then announced at the NATO summit eight months from now in Riga, Latvia.

For NATO to make this mission leap, there must be real European support and effective U.S. leadership. In principle, a more global NATO would pursue precisely the kind of goals embraced in Europe’s own security strategy. Defending Europe by dealing with these new threats is a core European – and U.S.-Canadian – foreign policy objective. A case in point is Iran, where policy is now weakened because it is divided among several institutions, no single one of which contains all the Western nations whose security is endangered by Tehran’s nuclear program. It should be stressed that involving NATO does not necessarily mean military action; it means, however, a seriousness of diplomatic and political purpose backed by the threat of collective action.

Europeans must have the political will and resources to participate in authorized missions in sometimes remote areas. This is not cheap, and the United States cannot always bear so much of the burden. Today only six of our 25 NATO allies spend more than 2 percent of gross domestic product on defense, although all have agreed to this benchmark.

Washington must reestablish confidence in the wisdom of American leadership and policy; this has been severely damaged in the diplomatic wreckage of Iraq. NATO and Washington will have to accept the realities of a stronger European Union, whose members are seeking a common foreign and defense policy.

Riga is the place to put the alliance on a new track. NATO must be prepared for the challenges of this century and produce a new definition of its raison d’etre. That is a nice French phrase for “reason to exist”; we hope that France, and its colleagues in the European Union, will see the value of a larger NATO role outside Europe. For that to happen, Washington will have to take the lead – for the third time in NATO’s history – in redefining the role of the alliance.

Mr. Holbrooke is a former U.S. ambassador to the United Nations. Mr. Asmus is executive director of the German Marshall Fund’s Transatlantic Center in Brussels.