LONDON (AP) -- If the European Union were a patient, its survival would be seen as threatened by multiple organ failure.
That's the view of many experts as EU leaders prepare for a Brussels summit that starts Thursday. Analysts believe the combined strain of challenges including a refugee crisis, threats facing the euro currency and Britain's plan to hold a referendum on whether to leave the EU may be unbearable for the 28-nation bloc.
Just 20 years ago, the EU seemed to be growing in stature as it proudly offered freedom and democracy — along with lucrative subsidies, military alliances and billions in foreign investment — to newly freed former Soviet satellites.
Now, NATO warships are steaming toward the Aegean Sea in an escalated bid to impose order on the chaotic arrival of more than 1 million migrants, which has not abated despite the wintry weather in southern Europe.
Informal mini-blocs have formed within the European Union, with some countries banding together to challenge, or just ignore, the EU's announced refugee resettlement program. Temporary border controls have been introduced in key countries including Germany and France, threatening the cherished notion of freedom of movement across European borders.
Britain, a nuclear power with a seat at the U.N. Security Council, is demanding concessions before a referendum on whether the U.K. should simply abandon the EU, a prospect known as "Brexit." And a slow-burning, extremely divisive budget crunch threatens the future of the euro single currency that has been a hallmark of European integration.
Ian Kearns, director of the European Leadership Network research group in London, said the EU is "undergoing an existential crisis" as a once shared sense of mission fades. Countries are pursuing their perceived national interests instead of seeking collective solutions, he said, and the notion of European solidarity is fading.
"It's anybody's guess now whether it will survive long term," he said of the European Union. "I think it's that serious. It's not just the migration crisis, or Brexit. The challenge is the lack of faith in the mainstream political class in Europe that is evident across the continent, manifested in the rise of populist movements. The migration crisis has simply highlighted it."
The summit is one of a series of meetings that have tried, but mostly failed, to find an effective collective response to the chaotic arrival of so many people. Leaders will consider fairly minor changes to Britain's status aimed at placating restive British voters ahead of a referendum, and assess how well — or poorly — earlier edicts on migration have been implemented.
The union has a knack for solving difficult situations by building consensus, and papering over cracks with layers of bureaucracy, but some warn the migrant situation is a more serious threat to continental unity.
Anand Menon, director of the UK in a Changing Europe group at King's College London, says the European Union simply doesn't have a practical method of tackling its myriad mounting problems.
The structures set up when the union was formed by six countries as the European Economic Community in 1958, and diluted with the addition of so many countries with differing perspectives, are simply too weak, he said, so nations either make unilateral decisions or forge small alliances with other countries in the bloc that share their concerns.
"The European project is probably in trouble," he said. "The EU is where it's been for the last few years: Very big crises without the tools to address them. It's a halfway house of integration. You have a little bit of authority in these areas — the migrants, Greece — but the big decisions are made by the member states. It's fragmented because the member states have completely different views."
Europe needs to have one cogent immigration policy to cope with the influx of people from the Middle East, Africa and elsewhere but won't be able to forge one because countries don't view the problem the same way.
"The countries in the south like Greece and Italy are facing the brunt of it," Menon said. "A few countries in the north — Germany and the Scandinavians — were generous at first and are now regretting it. The Brits are pretending it's not happening. And the Visegrad countries (Hungary, Poland, the Czech Republic and Slovakia) say they are not interested in helping for reasons of culture and history. They say they have no history of taking in migrants."
Officials had expected the flow of desperate people fleeing war and poverty would slow during the winter months, but The International Organization for Migration said this week that 76,000 people — nearly 2,000 per day — have reached Europe by sea since Jan. 1, a nearly tenfold increase over the same period the year before. More than 400 have died, most of them drowning in frigid waters.
In this diffuse environment, it is difficult to see the EU managing to respond effectively to such an unpredictable situation.
It was much easier 20 years ago, before Islamic extremism had showed its face inside Europe. The opening of the continent's internal and external borders was seen then as a welcome part of a peace dividend, not an Achilles' heel that left residents more vulnerable to suicide bombings and marauding gunmen. The relative stability in the Middle East meant the flow of migrants was manageable, not seen as a threat.
At the time, French President Francois Mitterrand and German Chancellor Helmut Kohl articulated forceful arguments in favor of more integration, and they were used to imposing their vision on the rest of the bloc, which was smaller and easier to manage.
When the Schengen Agreement was signed in 1985 it heralded a new era of passport-free travel in much of Europe, speeding trade, facilitating the easy movement of workers and students, and giving concrete, facts-on-the-ground reality to the idea of a continent turning its back on the wars of the past in favor of a more hopeful vision.
This inclusive approach guided the expansion of the European Union — as the bloc was renamed in 1993 — when Eastern European countries lined up to join.
Stefan Lehne, visiting scholar with Carnegie Europe in Brussels, said the unsolved refugee calamity may put the EU integration process into reverse by rendering the Schengen agreement unworkable — pointing out that border controls have already been temporarily reintroduced in some countries, as allowed by Schengen rules — and threatening other integration goals.
He said the rule of law, and the EU's authority, has already been undermined by the bloc's failure to implement an agreed upon quota system calling for the resettlement of refugees in a number of countries. Stark divisions have been exposed, he said, by the way the Visegrad countries in Eastern Europe reap the economic benefits of EU membership but while refusing to help the refugees.
All these factors, he said, have put the brakes on integration — and may shortly lead to its opposite.
"This is really the first time we might lose a very real achievement of the integration project, Schengen, with important economic costs," he said. "It's also very symbolically important. My sense is that unless we get a grip on refugees, the integration process will be reversed."