On Thursday, European leaders are gathering in Belgium to mark the 100th anniversary of the start of World War I — the bloodbath that ended millions of European lives.

And killed 116,516 U.S. troops. And laid the groundwork for World War II.

The centenary ceremony in Ypres, Belgium, provides a good reminder that whenever relations among European nations break bad, the rest of us need to pay attention.

It's time to listen up again.

That's because the Flanders Fields Commemoration will be followed by a fractious European Union summit on Friday. European leaders had been expected to use their time together to discuss strengthening their economies and responding to Russia's aggression in Ukraine. But much of their energy will instead go to scrapping.

In fact, their feuding already is so intense that some analysts fear this summit could mark the beginning of the United Kingdom's slide out of the EU. An exit would dramatically change the political union designed to spread peace and prosperity across the English Channel and the European continent.

"If the U.K. left, it would fundamentally change the outlook of the EU," said Jacob Kirkegaard, an analyst with the Peterson Institute for International Economics. "The EU would become more protectionist and less global" without the U.S.-friendly attitude of the British people.

"That would be a very bad outcome for the U.S. because what you (Americans) want is an EU that is outward looking" and open to transatlantic trade, Kirkegaard said.

He notes that the EU, with roots stretching back some 60 years, has faced crises before. But its leaders have always found ways to muddle through and then draw closer — and he thinks that will happen again.

That view is shared by Danny Alexander, chief secretary to Britain's Treasury.

"Despite seemingly increasing Euro-skepticism in the U.K., we are not on the brink of exiting the EU," Alexander said on Wednesday at the in Washington. "Doing so would be a disaster for Britain, putting our hard-fought economic recovery at risk."

Alexander said his latest Treasury analysis shows that leaving the EU could cost the U.K. 3.3 million jobs because of lost trade.

So if a breakup would be so terrible, why are we even talking about this?

Because U.K. Prime Minister David Cameron and many of his fellow countrymen are so unhappy with EU policies and regulations that they are loudly talking about a possible exit. They want the EU bureaucracy, based in Brussels, to become less intrusive in member states' domestic issues, including those involving the environment and immigration.

The U.K. has always been among the least satisfied members of the EU because of its historical orientation away from the continent and towards the U.S. and Canada. In 1975, U.K. voters held a referendum to decide whether to stay in Europe's common market, and most wanted to remain in. But the threat of a U.K. exit has continued to loom over the ensuing decades.

Now the long-simmering disagreements will come to a head Friday. But to better understand what's happening, let's first review some basics:

-- The EU is an economic and political agreement among 28 countries to allow for the free movement of goods, services, capital and people. Each member state still has its own government, but the European Parliament makes decisions affecting all participants and the European Commission serves as the executive branch, implementing decisions and upholding agreements.

-- The EU also has a European Council made up of all the heads of state. No nation has ever pulled out of the EU.

On Friday, the Council will nominate a Commission president. The person in line for the presidency is Jean-Claude Juncker. He served for 18 years as prime minister of Luxembourg, a speck of a country less populated than metro Toledo, Ohio.

In the U.K., Juncker is seen as an old-school politician, more of a fixer from a tax haven than a leader from a real nation. Juncker wants a tighter union, but Cameron fears a power grab by Brussels' bureaucrats and insists the Council pick someone else.

Other member-state leaders say Cameron must give in because rules are rules and Juncker is the next batter up, like it or not. Cameron could retaliate by campaigning for a British breakup in an in-or-out referendum that could be held in his country in 2017.

That threat has left European leaders fuming. For example, in a leaked, secretly recorded conversation with another minister, Radoslaw Sikorski, the Polish foreign minister, used many expletives to condemn Cameron's "stupid propaganda."

Europe's business leaders are worried because they don't want to lose their common market. The European Round Table of Industrialists says their key goal is "further integration of EU markets."

A big change in the EU makeup also would be a problem for the White House, which has been pushing a massive trade pact with the EU. The Obama administration would strongly object to a U.K. exit from the union.

So in coming days, Europe's leaders may exchange harsh words that will fray the ties of commerce and political solidarity. Or perhaps, amid the headstones and poppies in Flanders fields, the willingness to compromise will blossom again.

Alexander Privitera, an expert with the American Institute for Contemporary German Studies, believes Cameron has been using the exit threats to gain more bargaining leverage with his fellow Council members. In the end, the U.K. will allow the Juncker nomination to move forward, Privitera predicts. But he adds that Cameron also will extract compromises that move the EU in a direction the U.K. would like.

Privitera says most European leaders have made it clear that, in light of their continent's legacy of war, they don't like harsh confrontations and bitter breakups. And, he says, German Chancellor Angela Merkel has, in effect, reminded Cameron that, in this new century, "that's not the way we do it in Europe."

We hope.

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