Quentin Peel
Financial Times

The European Union is facing a momentous decision this autumn on whether and when to launch membership negotiations with Turkey. It is not just a question of whether the Union can absorb a very large, very poor and overwhelmingly Islamic state. That is difficult enough. It is also a decision that will determine what sort of global player the EU becomes in the 21st century.

Those who object to Turkish membership tend to have two main grounds for their argument. One is that it would amount to the "Islamisation" of Europe; the other is that it would make the EU hopelessly unwieldy and dysfunctional and halt the steady process of European integration.

The Islamisation argument is profoundly wrong-headed. It suggests that Europe is the same sort of bastion of Christianity that it was in the Middle Ages, holding the "infidel" forces at bay at the gates of Vienna. It is a backward-looking vision of Europe, defensive and even racist. What the members of the EU share today is a commitment to secular states, with freedom of religion for their citizens. Turkey shares that view, although it should do more to protect the freedom of minorities.

The second argument, that the continuing process of enlargement is eventually going to destroy the cohesion of the EU, is more persuasive. Perhaps the Union should have stopped growing with 10 or 12 members. All would agree on pooling sovereignty - not just with a single currency, but also harmonised taxation, social security, immigration policies and the like. This would be "core Europe". Britain would probably not be in it. Nor would most of eastern Europe and Scandinavia.

The reality is that the EU has left such hopes of a tidy, integrated political union far behind. It is now, with 25 member states, a very different animal - and it is not going to stop there. For the EU is the victim of its own success. Enlargement has been the single most successful policy of recent years, because it has stabilised an ever-wider area of Europe with the guarantee of democratic government and growing prosperity. Greece, Spain and Portugal were the first examples. The new member states from central and eastern Europe have confirmed that trend.

The list of emerging democracies seeking to join gets ever longer. The prospect of EU membership provides an extraordinary incentive for them to entrench democracy, fight corruption and gradually build a law-based system and market economy. Bulgaria, Romania and Croatia are at the front of the queue. It looks inevitable that all the countries in the Balkans will be offered the prospect and join eventually - including Serbia, Macedonia and Albania. Ukraine is not that far behind.

The EU cannot say No. But if all those countries are likely to join, the EU cannot honestly spurn Turkey's 41-year-old membership application, or impose special conditions. Both in terms of its economy and its democracy, it is already far better prepared than, say, Romania, Ukraine and Albania, to mention just three.

The borders of the EU cannot be defined on the basis of culture or religion. Geography is really the only objective measure of what is European. That would include Turkey and Russia as part-European, but exclude Morocco and Israel. It might be invidious, but at least it would be clear.

It would be a huge challenge to make such an extended EU function coherently. It would have to be a union of variable geometry, in which not all 30-plus members subscribe to every detailed rule or policy. It would have a single market and a common currency, free movement of people and capital and, with luck, a common foreign and security policy. The last may well be most difficult.

If Turkey is a member of such an EU, it would make the union a far more outward-looking global player than if enlargement halted in the Balkans. Turkey brings direct experience and involvement in the Middle East and - perhaps just as important - central Asia. It would also bring a huge contribution to any common European security policy, with its large army and long military tradition. It has a young population and a dynamic economy, too.

Outside the EU, by far the strongest proponent of Turkish membership has long been the US. Inside the union, one of the most doubtful is France. In terms of their own national interests, both may well be wrong.

The EU is already a serious economic competitor with the US, carrying as much clout in world trade negotiations. But it cannot compete in foreign policy and security terms, even if it wanted to, because of internal divisions and lack of military capacity.

With Turkey on board 10 or 15 years hence (it would take that long to finalise negotiations), the EU could become a much more serious global power. Of course, it would still have to get its act together. If it has not done so by 2015, it will not be Turkey's fault. But who can predict how far the process will have come in that time? A few more unilateralist administrations in Washington might do wonders in forging greater European unity.

It is a vision of a more multipolar world that many strategic thinkers in both Paris and Ankara probably share. Turkey is far less pro-American, and far more pro-European, than it used to be - thanks to the war in Iraq and US policy in the wider Middle East. An enlarged EU with Turkey on board might be a serious counterweight to US hegemony, or at least a partner that would have to be taken much more seriously than it is today.