By Stephen Mulvey
EU reporter, BBC News
The European Union has taken in two new members before it has recovered from the indigestion caused by its biggest-ever enlargement two-and-a-half years ago.
How Bulgaria and Romania handle farm aid will be under close scrutiny
In May 2004, 10 new countries joined the club, eight of them former communist states from Central Europe.
The unprecedented speed of this next expansion, albeit a much smaller one, partly explains why the reaction in many European capitals is, as one British columnist has put it: "Oh no, do we have to?"
"The indigestion from that biggest-ever enlargement is putting the whole enlargement process at risk," says Hugo Brady of the Centre for European Reform.
Some countries are still reeling from the scale of migration from these 10 new members, which no-one predicted.
The European Commission estimated there would be a total of 70,000 to 150,000 migrants per year throughout the EU, whereas the UK alone is now thought to have received up to 600,000 over two years.
JOINED IN 2004
No-one knows how many migrant workers will make the journey west from Bulgaria and Romania, in addition to those who have already come either illegally, or by obtaining work permits.
Romania's population is roughly half the size of Poland's, but it is double Hungary's or the Czech Republic's.
And it is much poorer - its GDP per head is a third of the EU average, compared with a half in Poland's case.
The older EU states have also found a couple of the newer states to be awkward partners at times.
Poland startled some EU governments by fighting tooth and nail over its voting rights in the EU's Council of Ministers before it had even joined.
More recently, it has argued for the re-introduction of the death penalty - anathema in many European capitals, raised hackles by bringing a party with a reputation for anti-Semitism and homophobia into the governing coalition, and vetoed the start of negotiations with Russia on a new strategic partnership agreement
Hugo Brady draws parallels with a "difficult period" which followed the accession of a "somewhat belligerent and nationalist" Greece in 1981, not long after it emerged from dictatorship.
The period did not last long, he says. EU membership helped to seal the country's full transition to democracy, and it quickly became a valued member of the union.
The indigestion problem, however, is more serious at street level than it is in the corridors of power.
Slap in face
Some Europeans have come to associate enlargement with a heightened risk of crime, and a source of economic problems from low wages to unemployment.
The fact that more than 60% of people in some European countries disapprove of further enlargement would be awkward enough at any time, but is particularly so in the wake of the rejection of the draft EU constitution by French and Dutch voters in 2005.
That big slap in the face from two founder members of the union hugely increased the pressure on the EU to show that it is democratic, delivering policies that voters want, and not a train running out of control.
A better analogy might be with an ocean liner which takes years to stop, as the EU committed itself to accepting Bulgaria and Romania a long time ago.
It has in fact committed itself to accepting all the Balkan states and Turkey, if and when they meet the membership conditions.
The EU has not yet fully explained how it intends to balance these future commitments against the opposition to enlargement in France, Germany and elsewhere.
Bulgaria and Romania join on 1 January 2007
Turkey and Croatia start membership talks in October 2005
Macedonia gains "candidate" status in December 2005
Albania, Bosnia, Kosovo, Montenegro and Serbia are expected to join in future
The message from an EU summit in December 2006 was that the rules for joining the EU would not change, but would be applied more strictly.
The commitments to the Balkan states and Turkey would be honoured, the leaders said, but they implied it could be a slow process.
Even the accession of Croatia - expected before the end of the decade - may be made conditional on changes to the EU's rulebook, such as those included in the ill-fated constitution.
After Croatia, even countries that satisfy the European Commission will face an additional hurdle - a referendum in France on the ratification of the accession treaty.
This hurdle was fixed in the French constitution before the referendum on the constitution, in an attempt to alleviate the country's enlargement indigestion. It didn't work.