By Olli Rehn, member of the European Commission responsible for enlargement

"The European Union took big strides towards further enlargement at the end of 2004. It closed accession talks with Bulgaria and Romania, which will allow them to join the EU in 2007 provided they are ready. And later this year, the Union will open accession negotiations with Croatia and Turkey.

Enlargement is one of the EU's most successful policies and a powerful foreign policy tool. The EU has progressively extended its zone of peace and democracy across the European continent. Following the entry of 10 new members last May, the EU stretches from the Atlantic to the Carpathian mountains, and from northern Lapland down to the coast of the Levant in the eastern Mediterranean.

After this "big bang", the EU needs to pace itself. Further enlargements must be gradual and carefully managed to ensure that European citizens support them and that the Union maintains its capacity to act. But the EU cannot close its doors. What principles should guide our enlargement policy?

I am often asked where Europe's ultimate borders lie. My answer is that the map of Europe is defined in the minds of Europeans. Geography sets the frame, but fundamentally it is values that make the borders of Europe. Enlargement is a matter of extending the zone of European values, the most fundamental of which are liberty and solidarity, tolerance and human rights, democracy and the rule of law.

The EU treaty says that "any European state may apply" for membership. According to the new constitutional treaty - signed last year but still awaiting ratification - "the Union shall be open to all European states which respect its values and are committed to promoting them together". But the established conditions for accession go beyond the treaty provisions. There are four prime criteria that a country must meet in order to win the acceptance of the European public.

First, a country must have a European vocation, as measured by the will of its own people to join. The Turkish people seem to have this vocation while the Norwegians seem to lack it, even though Norway could meet most of the other criteria today. Accession requires a concerted effort over many years, so political leaders must ensure their population broadly backs the enterprise. Joining the EU changes a country. It becomes part of a larger community, which affects its domestic laws and institutions. Elites alone cannot ensure the legitimacy of this decision; it must be backed by the citizens.

Second, any prospective member must respect European values and, most importantly, be able to live up to them. This concerns particularly the rule of law in all spheres of life - the litmus test for Turkey's accession to the EU.

Equally important is that countries relinquish power politics and spheres of influence as their modes of operation inside the EU. The greatest achievement of postwar Europe is that its governing principle is no longer "might is right". As Robert Cooper, the EU diplomat, so rightly concludes, in its internal business the EU has moved from the modern world based on balance of power to a postmodern world based on the rule of law. In this sense, the EU is a postmodern entity: a community of states that have agreed to pool their sovereignties and obey common laws in order to increase their impact in the world. These European values differ from the mindset of spheres of influence currently prevailing in Russia.

Third, because the EU is a community of law, all members must be able to carry the burden of membership, not just its benefits. Potential joiners have to show that they have the administrative capacity to apply EU rules properly and the political will to do so.

One of my priorities as enlargement commissioner is to ensure that all the countries queuing to join - starting with Bulgaria and Romania - are genuinely able to fulfil the commitments they make in their accession negotiations. It does the EU no good to welcome a country only to find that it cannot cope with the rigours of membership - and the new member will not benefit either.

A fourth and oft-forgotten condition for enlargement is the "Union's capacity to maintain the momentum of European integration". Paralysis would be too high a price to pay for expansion. Fortunately, history has proved it is possible simultaneously to deepen and widen Europe. In the past 15 years, the number of members has more than doubled and the Union has taken large steps towards deeper political and economic integration: the Schengen area of free movement of people, economic and monetary union, and a significant reinforcement of European foreign, security and defence policies.

Enlargement is not the EU's only policy for dealing with its neighbours. We are supporting the Ukrainians' quest for democracy by means of the European neighbourhood policy, which paves the way for free trade, political dialogue and human contacts - all of which are essential for closer integration with the EU. For the countries of the western Balkans - often weak states where institution-building is crucial - we have created a stabilisation and association process that includes the prospect of EU membership.

Although the borders of Europe are more mental than physical, geography still matters when it comes to spreading European values. My goal is that in 2009 the EU will have about 27 members, with half a dozen western Balkan countries well on their way to EU membership, and Turkey on track, through rigorous reforms. If these countries can fully adopt the EU's values, peace and prosperity in Europe will be more secure.

The writer is the member of the European Commission responsible for enlargement

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