The twin enlargements of NATO and the European Union in March and May 2004 respectively were momentous events in their own right. At the same time, they took place in a larger context: one of a profound – if gradual – reshaping of Europe's security landscape. Its most visible manifestations are the shift towards more autonomy from the United States and an intense debate over the circumstances in which force should be used to combat new threats. Lurking in the background is the question, arguably more open today than at any time in recent memory, of Russia's role in European and global security. Enlargement will have an impact on all three issues in ways that are not yet clear, but whose contours are already becoming visible. In turn, the final shape of the European Union's security architecture and its chosen strategy will determine NATO's future role on the continent.

The 11 September and 11 March attacks left all EU and NATO members more keen to act against new threats but not necessarily in agreement on when to use military force, against whom and under what conditions. There is a general consensus that early and "robust" use of force may occasionally be needed, but it sits awkwardly alongside a deep-seated preference for a society governed by international law in which the use of force is rendered unnecessary by the discipline imposed by multilateral institutions. All Allies would agree that ideally military action should be both effective and multilateral, but the relative emphasis that countries place on "effectiveness" as opposed to "multilateralism" differs, as witnessed in contrasting attitudes towards both the Afghan and Iraq campaigns.

Differences have spilled over into other areas of NATO's work as well. The NATO Response Force (NRF) was created to give the Alliance the ability to apply force rapidly when necessary. But its creation has not been without controversy. Agreement in principle to conduct more expeditionary warfare should not be confused with consensus on what to do if, for example, Iran goes nuclear, or if Syria is found to harbour terrorists. Only when the NRF is called on to act will we learn more about the outer limits of Allied cooperation.

Where do the new members fit in? Their words and actions speak in favour of the more assertive posture that has not always found universal acceptance in Europe. They seem to come down on the "effective" side of the debate rather than the "multilateral" one. The new members' militaries, too, are being rapidly retooled for the new missions. Indeed, in some ways, the newcomers are ahead of the established Allies in moving away from static defence postures. This is by necessity as much as by choice. The need for downsizing expensive Warsaw Pact-era militaries coincided neatly with the demand for leaner and more expeditionary forces.

To be sure, the new Allies' commitment to participating in new missions comes with reservations. The flip side of being both newly free and relatively poor is a tendency to take a narrow view of one's interests. Wars and reconstruction are expensive and the thought of spending scarce economic and human capital on improving somebody else's lot rings hollow in countries where the average income barely reaches 50 per cent of the European average. As accession countries know all too well, their new freedom of choice includes the freedom to stay out of "somebody else's" wars. "The need for global action is far from universally welcome in the Czech Republic," writes Czech security analyst Zdenek Kríz in a forthcoming book on EU enlargement and security. "A number of key parties and personalities prefer an essentially buck-passing strategy of leaving the responsibility to other, bigger and more influential countries."

Even so, the new Allies lean towards the assertive vision of security put forth by London and Washington. Their governments in general seem to take a bleaker, more Darwinian view of international relations than their Western neighbours. Strategic calculations also argue in favour of sticking close to Washington. Military power matters and the United States has most of it. As Hungarian researcher Tamas Meszerics writes: "[The new Allies'] realist geopolitical rhetoric may not be in sync with the language of the present-day European discourse... but it goes a long way towards explaining the willingness of many small and medium European powers to challenge the French and German visions of European foreign policy." The glue binding new Allies to Washington, in other words, is built on more than a sense of gratitude and moral obligation.

Whether enlargement will change NATO's policy toward Russia and if so, how, remains an open question. What is clear is that the event has transformed Russia's view of NATO, since Moscow has responded to enlargement with a mixture of scepticism and hostility. For NATO, Moscow's reactions have clearly raised the potential cost of getting Russia policy wrong.

The new members bring with them a different view on Russia and one that has not significantly been aired at NATO to date. Most of their political elites grew up with an intense distrust of and antipathy towards the Soviet Union, attitudes which, rightly or wrongly, have generally been transferred to Russia. Five years ago, fears that Poland's accession to NATO would cool the Alliance's relationship with Moscow proved unjustified. But these fears were based on a rather shallow reading of Poland's foreign policy vision. This, far from being Russia-centric, has consistently focused on carving out a respected role for Warsaw in Europe and a leading role in the region as well as on prodding Poland's Eastern neighbours – Belarus and Ukraine – towards democracy. A confrontation with Moscow might have undermined this policy. The priorities of the new accession countries' foreign policies are different with the result that they may not follow Poland's path.

On the other hand, NATO may benefit from acquiring a more finely tuned "Russia" radar, informed by the knowledge and experience of some of those countries that know Moscow best. One can safely assume that Moscow's continued search for its own identity will involve forays in both foreign and domestic affairs that most European states would deem unacceptable. On recent such occasions, neither the European Union nor NATO has succeeded in crafting a principled common response, one that might deter similar behaviour in the future. Russia's importance in the campaign against terrorism, its mineral resources, and its dominant role in the region simply offer Moscow too many opportunities to discourage criticism and to sow discord among its counterparts. This is a structural problem that plagues both EU and NATO relations with Russia.

The new members might help restore some balance to the relationship. The Baltic states in particular are far more focused on Russia than any of the old Allies, and also less likely to be willing to compromise. The challenge for NATO will be to avoid giving in to irrational fears while tapping into the energy and the focus of the new members' policies towards Moscow. Getting the balance right will be important, all the more so because of the attention that Russia has given to enlargement.

The defining event of spring 2004 may turn out to have had little to do with the Alliance directly. A few weeks after NATO enlarged, the European Union opened its doors to ten new members, including eight former communist countries. If these countries manage to make their voice heard in Brussels, they could play an important role in defining Europe's relationship with Washington and, in turn, the European Union's relationship with NATO.

The European Union, at least on the security front, has undergone a change no less dramatic than that of NATO. So-called "common" foreign and security policies were, until recently at least, essentially arrived at by collating the compatible parts of individual member states' positions. The EU foreign policy and security apparatus in Brussels exercised little influence over member states' policies and was not in a position to create policies of its own.

This situation is currently changing and the focus on creating policies is slowly shifting to Brussels. The EU Security Strategy produced in December 2003 serves as a good example. It was still largely built by mining the various member states' policies for common positions, but for the first time, in a modest but symbolically important way, it pushed a number of EU countries towards a security philosophy which they might not have embraced of their own volition. The document has been described by an insider as 90 per cent descriptive and 10 per cent prescriptive. While 10 per cent may not appear much, it would have been unthinkable a few years ago. It seems to herald the era of mixed responsibility for European defence, with policies still mostly made in and implemented by member states, but increasingly circumscribed and sometimes prescribed by interests defined at the level of 25.

What is not yet known is the exact form of the future Brussels contribution to this process. To some EU members, European Security and Defence Policy (ESDP) represents a "Plan B", to be used in cases when the United States cannot or does not want to become involved. To others, less America in Europe is the whole point. For yet another group of countries, EU foreign and security policies are an institution-building exercise with little relation to defence.

These competing visions conceal very different outcomes for the future shape of relations between the two sides of the Atlantic, and the eventual result could be a profoundly different EU-NATO relationship.

The new EU members come to the debate over Europe's security policy late. They were largely absent from earlier discussions when many of the blueprints for ESDP were drawn up. Indeed, in 2003 alone, the European Union adopted its Security Strategy, inaugurated a Rapid Reaction Force and launched its first two military missions.

But the new members' sideline narratives consistently stressed the indivisibility of EU and NATO roles. In practical terms, "indivisibility" tends to translate into coordinated threat assessments, preference for joint operations and a common set of planning standards for the expeditionary portions of countries' armed forces. It implies the continuation of the transatlantic alliance in its real sense – doing things for others that one would not normally do.

The new Allies are most clear on the need for an ongoing US role in European security. The EU discourse on this point covers quite a range, from heartfelt desire to keep NATO and the United States in the centre of Europe's defence plans to suggestions that the United States could – or perhaps should – leave the business of defending the continent. The new Allies seem firmly and unequivocally to favour the former vision. "It is in Slovakia's interest (as well as in the interests of EU accession countries) to make sure that NATO is still seen as relevant from the point of view of the United States," writes Slovak sociologist Olga Gyarfásová. "If the ties between the United States and the rest of the Allies are weakened, this would also lead to weakening of the security guarantees in which Slovakia has invested so much political capital."

While the new members have articulated their views clearly, their ability – or even their desire – effectively to influence the European Union's security agenda is an open question. The governments in Bratislava, Budapest, Prague and elsewhere have tended to view EU integration as essentially a passive process, one of identifying EU consensus and reshaping their policies to fit the mould. On many issues, however, and on defence in particular this is the wrong approach. Europe's security identity is only now being formed and the EU and NATO enlargements present a unique opportunity for the new members to help shape it.