By Chris Morris
BBC Europe correspondent
Europe took a huge decision this week.
Thousands of supporters greeted Turkey's PM on his return home
There are still years of negotiations and plenty of pitfalls ahead, but if Turkey ever joins the European Union, people will look back on this summit as a decisive moment.
An EU with Turkey as a member will look quite different from anything the founding fathers of the union ever envisaged.
The summit was fractious, tense and often grumpy.
Harsh words were exchanged behind the scenes, and at one stage several EU delegations thought the Turks were going to walk away from a deal.
Even after agreement was finally reached, the Turkish prime minister and his Greek Cypriot counterpart had a verbal bust-up in front of their colleagues.
Welcome to the next 10 years.
Dictation of terms
This is the most difficult and ambitious enlargement of all.
Yes, allowing most of the former communist states of eastern and central Europe to join together in one fell swoop this year was a massive undertaking.
TURKEY'S DRAFT EU ENTRY TERMS
Turkey must sign a customs accord extending to all EU members, including Cyprus
The accord must be signed by the start of entry talks, proposed for October 2005
Membership talks will be open-ended
There is no guarantee of full membership if conditions are not met
If negotiations do fail, Europe will not turn its back on Turkey
Turkey must continue with political and economic reforms
Some safeguards may remain over migration of workers from Turkey
The EU has redrawn the map of Europe already.
But Turkey is different. It is the biggest, poorest country ever to be invited to start talks, and the most culturally challenging.
A Muslim country bordering the Middle East and the Caucasus; a country undergoing rapid political and economic upheaval, which has changed profoundly in the last few years and which will have to do much more of the same to be ready for membership.
For Turkey, it could be a steep learning curve.
Negotiations on joining the EU are not really negotiations at all. They are a dictation of terms.
Either you fulfil the EU's requirements, or you do not join. It is as simple as that. And Turks do not take that kindly to being told what to do.
Just the beginning
Will Turkey ever become a member?
It is possible that it could jump through all the hoops set up by the EU and then decide it does not want to join after all.
By transforming itself so thoroughly, Turkey could gain the confidence to strike out in a different direction.
Anyway, who knows what Turkey will look like in 10 or 15 years' time?
Who knows what the EU will look like then as well?
The process begun by Europe's leaders in Brussels this week will have to be completed by the politicians of the future.
But one fact was at the back of everyone's mind at this summit - no country which has begun membership talks has ever been rejected by the EU before.
History was certainly made this week. But the arguments on both sides about whether it was a step in the right direction have only just begun.