The news last night that Turkey has accepted the European Union's proposals to begin accession talks is dramatic. Coming after a tormented and gruelling 24 hours of negotiations on a complex set of issues, the simple fact that the talks are to go ahead is the main point to be grasped. It represents a notable success for the current UK presidency.
Turkey has now to adopt the EU's huge body of law and values. That is a monumental task. The shared objective is full accession, probably in 15 or so years' time. There will be a constructive - and progressive - opportunity for both sides to decide if this is a feasible objective.
In so deciding, Europe can demonstrate to a changing world that an alliance of civilisations and a creative dialogue between them is central to its own identity. Turkey has been part of the European political, human and cultural landscape for many hundreds of years.
It has helped to determine Europe's historical destiny in ways that transcend changing geographical boundaries. This decision keeps open that dialogue and reinforces the momentum of mutual adaptation to a changing world.
Turkey has for a long time been preparing for this moment and will be a long time negotiating a full accession agreement with the EU. It stands many years behind Europe's standard of social, educational and equality policies. Last night's opening session is thus a major stepping stone for both sides.
That it has happened is, however, no guarantee of a successful outcome involving certain Turkish membership. Even if full agreement is eventually reached, there will be referendums in France and Austria, the most sceptical member-states, on the question. Austria yesterday withdrew its reservations about the terms of reference for the Turkish talks, partly because the way was cleared for Croatia to open accession negotiations - a welcome development.
In its search for EU entry, Turkey has already made massive, if sudden, strides towards liberal democratic norms in its domestic politics and constitutional ethos, alongside its extensive economic and administrative adaptation. But continuing pressure to reform Turkey's centralised character will be exerted as negotiations proceed. Turkey must change as a result.
Coming to terms with its Muslim traditions and culture should help Europeans to adapt to one of the contemporary world's major challenges. Doing so can create a positive feedback from the Middle East, North Africa and Asia, tempering or rejecting trends towards civilisational conflict in favour of engagement.
This is in addition to the major economic benefits to be found in Turkey's home market and demographic buoyancy as well as in the security benefits arising from its window on the Balkans and the Middle East. Hostility towards Turkish accession, whether it is based on conviction, prejudice or ignorance, can shift - but only if political leaders are prepared to argue the case for Turkish accession with the same conviction they showed yesterday. That will be the real test of the next decade.