Virginia Mayo, Associated Press
European Council President Herman Van Rompuy speaks during a media conference at an EU summit in Brussels on Friday, Dec. 9, 2011. European leaders are wrestling over how much of their sovereignty they are willing to give up in a desperate attempt to save the ambitious project of continental unity that grew from the ashes of World War II. At stake at the summit in Brussels, which began Thursday evening, is not only the future of the euro, but also the stability of the global financial system and the balance of power in Europe.

BRUSSELS — Croatia on Friday signed an accession treaty to join the European Union in 2013, a move that comes just as the bloc is deep in an existential crisis of unprecedented intensity.

EU president Herman Van Rompuy welcomed the signing, saying Croatia will be an "active observer" in all EU forums until it becomes a full member 18 months from now. Its membership must still be ratified by the legislatures of the bloc's 27 member nations.

"Today is a historic day for Croatia and the EU. Croatia is set to become the 28th state of the union," he told EU leaders at the signing ceremony.

Critics say the Croatian government's decade-long effort to join the bloc could easily turn out to have been a giant waste of time. Many analysts have warned that time is running out for the 27 countries to find a way out of a financial crisis that threatens to unravel the 53-year old effort to integrate the European continent.

Following a marathon all-night session at an emergency summit, most leaders decided to back a new treaty with strict oversight over national budgets, as they tried to convince markets that the euro has a future in the wake of a crippling debt crisis.

But, Germany and France were unable to persuade all 27 European Union countries to agree to the treaty changes, in large part because of Britain's refusal to give up some powers.

"It's very, very odd for someone to join a club the night after the worst bust-up in that club's history," said Nigel Farage, a staunchly anti-EU British member of the European Parliament.

In terms of wealth, measured by GNP per capita, Croatia ranks in the upper half of the 12 countries that have joined the bloc over the past decade. Before any new member fully joins, the EU requires nations to fulfill a set of strict economic and political conditions that are addressed during the drawn-out accession process.

Croatia's entry talks lasted seven years and were held up repeatedly due to territorial disputes with neighboring Slovenia and demands that it arrest remaining war crimes suspects.

"Today Croatia is entering Europe, but more importantly Europe is entering Croatia," Croatian President Ivo Josipovic told the heads of government of all 27 members.

Croatia will become the second nation from the former Yugoslavia to join the EU after Slovenia, which became a member in 2004. All other countries that emerged from the Yugoslav federation — Bosnia, Kosovo, Macedonia, Montenegro and Serbia, as well as neighboring Albania — are also seeking membership.

Meanwhile, officials said EU leaders may postpone making Serbia a candidate for the bloc until their next summit in March.

According to a draft resolution, Serbia would have to normalize relations with its former province of Kosovo, which declared independence in 2008, said the officials who spoke on usual condition of anonymity.

Serbia had been widely expected to gain candidate status after extraditing the last of several dozen war crimes suspects earlier this year. But a recent outbreak of violence in northern Kosovo, where hard-liners among the Serb minority have blocked roads and clashed with NATO peacekeepers, appears to have put that on hold.

"EU leaders are just too busy with the euro crisis to waste too much time discussing Serbia's status," one of the officials said.

Meanwhile, tiny Montenegro was likely to receive the green light to open accession negotiations next June on condition that it continues a crackdown on organized crime and corruption.

The nation of 650,000 people received candidate status a year ago.