The European Union has a rendezvous with history this week. Turkey will be invited to begin talks over the course of next year that will probably lead to its membership in the European Union within about 15 years.
Turkish Prime Minister Recip Tayyip Erdogan (left) in Brussels last week
Turkey's entry will mark an epoch-making change, a merger of Christian Europe with the major Muslim power on its doorstep.
It will also push out the EU's territory to the borders of Iraq, Iran and Syria.
The decision could prove as momentous for the 21st Century as the Yalta Conference, which sealed the division of postwar Europe in the 20th Century.
Behind the smiles in Brussels, EU leaders know they are launching a geopolitical game with high risks.
This decision will change the character of the EU. Britain is among Turkey's strongest advocates. But many leaders in its founding nations, like France and Germany, fear the end of a cherished dream - to make Europe into a united superpower which could balance the dominance of America.
So the EU's coming political battles over Turkish membership will be fierce.
The first will be over the choice of words at the summit. Turkey will probably face special conditions:
Extra proof, before the talks start, that Turkey is carrying out key reforms, like the elimination of torture
A clause saying that its membership negotiations may be halted if its commitment to democratic reforms falters
A promise, in due course, by Turkey to give official recognition to the government of Cyprus, which joined the EU in May this year
Strict limits, for many years even after joining, on the right of Turks to live and work in other parts of the Union.
Valery Giscard d'Estaing is sceptical of Turkey's membership bid
France, backed by Austria and others, questions if Turkey will ever fit in with the political values and traditions of the European Union. Turkey's presence will make it harder for France and Germany to remain the masters of the EU project.
Valery Giscard d'Estaing, the former French president who chaired the EU's Constitutional Convention, said bluntly that the entry of Turkey, as an Islamic and mostly Asian power, would spell "the end of Europe".
Public opinion across the EU is lukewarm to letting Turkey in. In France 75% of voters oppose it. President Jacques Chirac has promised the French the chance to say yes or no to Turkish membership after it has completed the long negotiations.
And Angela Merkel, the current leader of the opposition Christian Democratic Union in Germany, wants Turkey to be offered a "privileged partnership" rather than full membership.
On the Turkish side, support for joining the EU is overwhelming. Turkey has long been accepted as a vital member of the western alliance, Nato.
And the government sees a place in the EU as fulfilling the ambition of Kemal Ataturk in the 1920s to make Turkey a modern, secular European state.
Turkey's Prime Minister, Recep Tayyip Erdogan, has warned the EU leaders not to play games with Turkey's hopes. It would be a terrible mistake, he says, for the EU to behave as a "Christian club". And if its members were to turn Turkey away "history will not forgive them".
Mr Erdogan has shown bold leadership in bringing Turkey to this point
Despite his own Islamist background, Erdogan has revamped Turkey's laws to align them with EU membership since his Justice and Development Party came to power in 2002. The biggest changes are:
Ending the army's power to intervene in politics. It forced out feeble or Islamist governments four times between 1960 and 1997
Enacting dozens of new democratic and civil rights laws, including a ban on torture and the death penalty
Tackling corruption and ending Turkey's chronic economic instability
Supporting April's United Nations plan to unite the Turkish-speaking north of Cyprus with the southern, Greek-speaking side. The goal of a political settlement before Cyprus' entry to the EU failed because of a "no" vote in the officially-recognised south.
Human rights groups insist that Turkey still falls far short of European standards. They cite the lack of real protection for the rights of Kurds, after decades of armed conflict, as well as practices like police torture and honour killings.
A bargain for Europe?
Still, Turkish membership in the EU now offers Europe a grand bargain. It could strengthen Europe's defences against Islamic terrorism, disprove the theory of a "clash of civilisations" between Islam and the West, and add weight to the EU's aspirations to be a global power.
Against that, the EU is taking on these risks:
The admission of Turkey may heighten tensions in the EU over immigration and the activities of radical Islamic groups
Turkey will change the EU in unforeseen ways. The French government admits it fears losing next year's referendum on the EU constitution if that issue is mixed up in voters' minds with that of Turkish membership.
By the time it is ready to join, Turkey's population will be well over 80 million - the largest in the EU. And Turkey is much poorer than any current EU members. So Europe may have bitten off more than it can chew.
But Prime Minister Erdogan is confident that Turkey will show that Islam and democracy can live together.
Asked whether his country was looking for a marriage of convenience or of love with the EU, he replied: "I want a Catholic marriage - one that lasts forever."