The relative political calm that has prevailed across the Atlantic this year will soon be tested by an ambitious U.S. effort to remake NATO into a global security organization able to go anywhere and do much more than fight wars.

Light sparring is underway behind closed doors over the sites and the agendas for alliance summits in 2006 and 2008 that could become essential components of President Bush's legacy in foreign affairs. The tactical arguments are precursors of a great strategic debate that lies ahead over the nature of global power in the 21st century.

This debate, if handled correctly, could enhance rather than damage allied unity. It encompasses the recent bitter transatlantic differences over the war in Iraq, which have been put aside for the moment. But the discussion to come must be far broader than Iraq, since it involves an underlying conflict over the direction of history as well as the direction of NATO.

Outwardly, transatlantic relations have improved substantially. A new German government that does not owe its electoral legitimacy to opposing Bush's policies has taken power in Berlin. France's drive to limit U.S. hegemony abroad has been weakened by internal problems. Secretary of State Condoleezza Rice has turned U.S. sniping at the European Union's negotiations with Iran over nuclear arms into meaningful support for that effort.

"We are in complete agreement on the goals and the means of resolving the Iranian nuclear question," Frank-Walter Steinmeir, Germany's new foreign minister, told me shortly after he met with Rice here last week. "The enrichment of uranium within the borders of Iran is unacceptable to us all, and we are offering a reasonable alternative to Iran . . . .That unity will continue . . . whether Iran seizes this opportunity or not."

But the Bush national security team continues to see a world being swept by radical changes that must be mastered and channeled, while many in Europe see a world standing still -- that is, possessing a rough strategic equilibrium that must be maintained through gradual evolution.

Structurally, this difference shows up in the implicit creation of an alliance within the alliance: Bush's America, Tony Blair's Britain, Silvio Berlusconi's Italy and the formerly Soviet-occupied lands of Central Europe and the Baltics that have deployed troops to Iraq. They constitute a politically coherent group committed to advancing democratic freedoms abroad, through military means if necessary.

But the former Soviet satellites once identified by Defense Secretary Donald Rumsfeld as "New Europe" have relatively less to offer in personnel and hardware for the alliance's military missions, even if their own recent enslavement makes them more eager to take part in those endeavors.

While their electorates resist committing resources to military spending, France and Germany nonetheless contribute forces through NATO to the pacification efforts in Afghanistan and to the alliance's new Rapid Reaction Force in much greater numbers than can New Europe. A key point of change sought by Washington is to have the organization as a whole pay for missions that are now funded only by the countries that take part in them.

Resolving the emerging cleavage between mission affinity and mission capability within the alliance is the key task for the president in his final two outings on the NATO summit stage. These gatherings, normally ceremonial rather than substantive, can redefine and revitalize NATO for the future, or consign it to history along with the Cold War, which created the organization.

The White House supports Latvia's bid to host the 2006 gathering at a date still to be determined. Some European capitals want the meeting held at NATO headquarters in Brussels instead, to avoid risking Russian displeasure and reaction over a NATO summit being held in a former Soviet republic.

There is also concern among European diplomats about an American push to use the 2006 summit as a moment of "transformation" for NATO into a global alliance that can take on large-scale humanitarian, reconstruction and peace-building missions around the world, rather than remaining close to its original purpose of the collective defense of Europe.

A glimpse of one possible mission for new NATO came in September when the fledgling Reaction Force ferried symbolic relief (10 tons of cots, tents and other supplies donated by the Czech Republic) to victims of Hurricane Katrina. Then, in October, the force mounted a much larger emergency air bridge to move 2,000 tons of supplies and 300 NATO troops to help earthquake victims in Pakistan and Kashmir.

Old Europeans may grumble that NATO should not be turned into the Red Cross -- that the alliance's redefinition should not become endlessly elastic. But Europe must then offer its own concepts for an effective new alliance that contributes to global security, rather than one that stands still and complains about American unilateralism.

That in turn should force the Bush administration to treat transatlantic differences not as a simple matter of political will -- of some alliance members having it and others not -- but as a matter of necessity to base a new NATO on a new common program that brings clear benefits to all 26 members.