In May 2004 the European Union of 15 expanded to 25. It was the most ambitious and challenging enlargement of the European project since the signing of the Treaty of Rome 47 years ago.
EU enlargement was celebrated across the new member states
The flag-waving and fireworks stretched from Tallinn to Valletta.
Politicians indulged in grandiose rhetoric about healing Europe's Cold War wounds, and for once they looked as though they meant it.
Yes, there were sceptics in the new member states, but by year's end even the notoriously grumpy Polish farmers were showing signs of warming to the reality of EU membership.
But now comes the really hard part. The EU is an unwieldy amalgam of 450 million people, 25 nation states, and 20 official languages - it is sorely in need of a makeover to boost efficiency, accountability and comprehensibility.
And the solution, of course, lies in the painstakingly constructed Constitutional Treaty for the Enlarged Europe.
Or does it?
Well, in 2005 the viability of this "blueprint for the EU's future" with its proposals for an EU president and foreign minister, a charter of fundamental rights and an expansion of qualified majority voting, will be tested.
Europe's leaders signed the constitution with all the self-importance of founding fathers in Rome last autumn, but it will only come into effect if it is ratified by every member state.
There will be referendums in at least nine countries starting with Spain and Portugal.
Both countries are expected to voice strong approval.
Much less predictable are the subsequent votes in the Netherlands, Denmark and France, to name but three.
France is a particularly intriguing proposition. Jacques Chirac and the right wing UMP strongly support the constitution, which was, after all, heavily influenced by former French President Valery Giscard d'Estaing.
Blair could face a tricky task to ratify the EU constitution
The socialists, too, have grudgingly decided to back it.
But there are powerful voices in France who see the new constitutional plan as a "sell-out" of their vision of a highly integrated "Social Europe".
It's just possible that the French public - perhaps airing unrelated grievances about the economy and immigration - might choose to give President Chirac a bloody nose by saying "non" to the constitution.
And then there is Britain.
The British government will hold the six-month long rotating presidency of the EU in the second half of 2005, so the UK referendum isn't likely until spring 2006.
Cynics suggest that Tony Blair (should he be re-elected) would breathe a sigh of relief if the French were to reject the treaty and thereby kill it off.
But failing that, the British public will find itself the focus of much EU attention by the end of next year.
Would Britain really reject the constitution even if all other members had accepted it?
And what would that mean, both for the EU and for Britain's future in the Union?
The EU is already under new management as a result of the five-yearly change in the executive branch, the Commission.
Its new President, Jose Manuel Barroso, had perhaps the briefest political honeymoon in EU history when his Commission team was unpicked by the European Parliament before it had even begun its work.
Parliamentarians hailed the "Buttiglione affair" as a triumph for democracy and accountability in the Union's institutions.
The end result was a Commission which has a distinctly more liberal (in the economic sense) taint than many previous teams.
And its big challenge next year will indeed come in the economic sphere.
For the Commission must work with the European Council to come up with an agreement on the next seven-year budget for the EU.
Britain, backed by the Germans, Dutch and several others, want to see spending limited.
They are fed up with the drain on resources represented by agricultural and regional subsidies.
But many other members - poorer, or with a bigger agricultural sector - accuse the EU's net contributors of selfishness.
There is the prospect of a classic EU fight - between big countries and small, new members and old, free marketeers and interventionists.
And beyond it all is a basic challenge for Europe's leaders - can they individually and collectively launch the reforms necessary to galvanise Europe's economy and make good their promise to create the most dynamic, growth-oriented economic zone in the world?
Which brings me to Turkey.
In October 2005 formal negotiations are supposed to start on Turkey's long-sought accession to the European club.
Just getting to the starting line has taken four decades.
Many welcomed Turkey's EU entry deal - but it faces some thorny issues before joining
Given its size, its relative poverty and its Muslim population, taking on Turkey will be the most challenging enlargement project in the EU's history.
Turkey has obvious strategic and economic potential, but in a year likely to be marked by increasingly strident discourse about the place of Islam in Europe and the dangers of a "clash of cultures", there may be growing scepticism in some quarters too.
Cyprus could also be a sticking point.
Is Turkey ready to make gestures pointing towards recognition of the Greek-Cypriot government?
The EU has swallowed one major expansion, and is considering another even more ambitious.
Its internal structure is ripe for reform.
In short, 2005 will see decisions taken which will do much to shape the Union's future identity.