European leaders are due to decide in December whether to begin accession talks with Turkey, a move that would extend the EU's frontier into the heart of the troubled Middle East, to the borders of Syria, Iran and Iraq, and the Caucasus. No greater issue looms on the EU's horizon.
For most of the 10 new members, joining is the logical consequence of the tearing down of the Berlin Wall 15 years ago. It confirms their place at the table of Europe's advanced democratic capitalist nations.
But the removal of the Iron Curtain between the free and Communist worlds has switched attention to an older fault line that is re-emerging between the West and the rest; Harvard professor Samuel P Huntington has dubbed it the clash of civilisations.
Turkey straddles two continents. But the bulk of its land mass has historically been called Asia Minor; its 68 million population is overwhelmingly Muslim; and the debate about its suitability as an EU member raises the most fundamental question about what Europe means today: does it represent a place or a set of political values? If the latter, should it one day embrace friendly reforming countries in north Africa and the Levant, turning the Mediterranean once more into a "European lake"?
The question was controversially raised by Valery Giscard d'Estaing, who said that if Turkey joined it would spell "the end of Europe". The former French president, who led efforts to develop a constitution for the new 25-member EU, said of Turkey: "Its capital is not in Europe; 95 per cent of its population live outside Europe. It is not a European country." He was immediately decried for portraying Europe as a "Christian club", when existing members such as Britain and France have millions of Muslim citizens.
Turcophiles said his claims were absurd. Istanbul, formerly known as Constantinople, was the capital of the (Greek-speaking, Christianised) Eastern Roman and later Byzantine empire for hundreds of years. The Ottoman empire played a central role in the affairs of the continent for six centuries - for much of it controlling the Balkans and twice threatening the gates of Vienna - until by its end Turkey earned the sobriquet the Sick Man of Europe.
Voltaire, the 18th century French writer and philosopher, praised the Ottomans for allowing ethno-religious cultures within the empire to run their own affairs. He said: "The great Turk is governing in peace twenty nations from different religions. The Turks have taught the Christians how to be moderate in peace and gentle in victory." But today there is no such admiration for Turkish political practice. One of the biggest obstacles to entry has been the country's poor record on human and minority rights.
Fashioned from the rubble of the Ottoman Empire by Mustafa Kemal in 1923, the Turkish state was modelled on Western-oriented European democracies.
But it was stuck in a human rights time warp until after it became an EUcandidate in 1999.
Opponents of Turkish entry point to its repression of its 12 million Kurds (including the torching of Kurdish villages in the 1990s, the imprisonment of Kurdish MPs and the denial of education in Kurdish), its continuing occupation of northern Cyprus, and its underdeveloped democracy (the government is still answerable to a National Security Council dominated by the military, which staged three military coups in the 1960s and 1980s and forced an Islamist-led coalition out of government in 1997).
There is also the economic problem. Turkey's population is large and, by European standards, poor: it could become a burden on the EU. The same was said, however, about Spain, Greece, Portugal and Ireland when they joined, and the EU context has helped them modernise - turning Ireland's economy into the Celtic Tiger.
The Turks have defended the position of the army in the past on the imperative of keeping the country intact in the face of Kurdish uprisings and, during the Cold War, the threat from the Soviet Union and its bitter rival, Greece. The army's position was reinforced by the military alliance with the United States and its key role in NATO.
PBut under pressure from the EU to meet the criteria of membership, prime minister Recep Tayyip Erdogan has shepherded a mass of reforms through parliament which, if implemented, will change Turkey into a modern European democracy. Gone is the death penalty; the military's role is being down-graded; many, though not all, minority rights - state education in the mother tongue, for instance - have been granted to the Kurds; and a serious effort has been made to solve the Cyprus problem.
Privately, EU officials say if the reforms are shown to be working on the ground, European heads of state will welcome Turkey on board (although the French may disagree). That would send a powerful message to the conflict-prone world of the Middle East and beyond: that Muslim and Christian nations can flourish economically, culturally and politically in a partnership inside Europe.
That would also set Europe apart from the US, in offering an alternative inducement to change to the use of overwhelming force. It is called persuasion - and in Turkey's case it seems to be working.
Whether Europe can continue expanding without becoming so big that it can't function is a moot point, but as Norman Davies, the eminent historian, has noted, denying a reformed Turkey entry on the grounds that she is a Muslim country would be an un-Christian act and out of kilter with the diversity that Europe represents today.
"We are building a new Europe in the 21st century, not in the 16th," he said.
TES international editor Brendan O'Malley is an associate researcher on Mediterranean and Eurasian studies at Kingston University and co-author with Ian Craig of The Cyprus Conspiracy: America, Espionage and the Turkish invasion (IB Tauris, 1999), which was shortlisted for the Orwell prize for political writing