LONDON • United States President Barack Obama has joined European leaders in their struggle to deflect a dagger aimed at the heart of the continent's political unity.
It is the combined impact of home-grown terrorism, mass migration from the Middle East and Africa, sluggish economic growth and a groundswell of nationalist sentiment.
Speaking at a press conference with British Prime Minister David Cameron on Friday, Mr Obama said European unity "is under strain".
He offered hope that "the ties that bind Europe together are ultimately much stronger than the forces that are trying to pull it apart".
The news conference came on the first full day of a visit to the continent that also will include a summit in Germany tomorrow with leaders of Europe's four largest economies.
A fracturing of the 28-nation European Union (EU) or a descent into dysfunction would undermine growth prospects for an economic bloc that is America's largest trading partner and would threaten cohesion among the American bedrock allies.
Mr Obama published an opinion article in the Daily Telegraph newspaper shortly after landing in London on Thursday, urging British voters to reject the so-called Brexit. He amplified his message in the news conference, pointing to the relative peace and economic prosperity Europe has enjoyed since World War II. Europe in the 21st century "looks an awful lot better" than in the 20th century, he said.
He also issued a veiled economic threat, warning Britons to be wary of claims made by Brexit backers that the US would quickly reach a trade agreement with Britain to maintain preferences the country gets as part of the EU. "Maybe at some point down the line there might be a UK-US trade agreement, but it's not going to happen any time soon," Mr Obama said. "The UK is going to be at the back of the queue."
Mr Obama arrived in London well aware of the delicacy of a foreign leader taking sides on matters that are essentially internal European issues.
London Mayor Boris Johnson, who favours leaving the EU, told the Associated Press "it's paradoxical that the United States, which would not dream of allowing the slightest infringement of its own sovereignty, should be lecturing other countries about the need to enmesh themselves ever deeper in a federal super-state".
Mr Obama approached the subject gingerly, saying he did not come to "fix the vote" but address a subject in which the US has a "deep interest".