Barack Obama has a full schedule in London this week. There is lunch with the Queen, on the occasion of her 90th birthday. There is dinner with the Duke and Duchess of Cambridge, otherwise known as Will and Kate. There are talks with the prime minister, David Cameron, about the appalling state of the world. And then, perhaps, with Cameron’s approval, the president may pronounce a few sentences encouraging the British to stay in the European Union. In mere anticipation of these words, Obama has already been attacked by anti-E.U. campaigners as “nakedly hypocritical” and the “most anti-British president in U.S. history.”

Indeed it is unusual for a sitting U.S. president to intervene directly in an election in a foreign country, and maybe risky. But then, there is nothing “usual” about Britain’s E.U. referendum, scheduled for June 23. Unlike in most elections, British leadership inside Britain will not be at stake. Whether the country votes “remain” or “leave,” the Conservative Party will go on running the country. But British leadership in the world is very much at stake. And because it really is a matter of profound, bipartisan, long-term U.S. interest that Britain remain a European power and thus a world power, Obama is right to take the risk and say so.

This is not merely because of the economic turmoil that could follow a rapid “Brexit,” a possibility recently flagged by eight former U.S. treasury secretaries in the Times of London. Nor is it just because pro-European Scotland might once again try again to divorce itself from the United Kingdom in the event of a “leave” vote, or that the Northern Irish settlement might also be disturbed. It is because Britain, outside the E.U., will lose any ability to shape the way Europe is run and regulated. It will lose its voice in the European economic, political and foreign policy councils where it has so often played a central — and a pro-American — role.

Britain, remember, was crucial to creating European sanctions regimes in Iran and Russia. Britain backed the integration of the eastern half of the continent. Britain has successfully pushed for European antitrust laws that have made the whole continent a friendlier place to do business. Britain has helped knock down entry barriers to trade and make the European single market real. When they push hard, which they don’t always, the British usually win their arguments — which is good for them, good for Europe and good for U.S. companies, too.

If Britain leaves, there is a risk that the rest of Europe could drift off in a not exactly Western direction. As the crow flies, Moscow is closer to Berlin than Washington. Worse, there is a risk that, outside the E.U., Britain itself would drift into the role of an offshore Switzerland, becoming a kind of amoral trading power, one of those countries with no friends, only interests. Many in London already think it’s more important to court China than to invest in NATO or worry about the retreat of democracy. The City, London’s financial district, is already home to a large number of lawyers, accountants and real estate agents who make fortunes helping autocrats hide their money offshore, creating shell companies and using them to purchase, anonymously, very large Mayfair houses. Once outside the E.U., unattached from the central political arguments taking place in Europe, this powerful class of people might well start promoting an apolitical Britain far removed from the Churchillian or Thatcherite nation of distant memory.

At a "Vote Leave" rally on April 16, London's mayor Boris Johnson encourages Britain to make the change and leave the European Union. He said, "It's now or never." (Reuters)

If Britain leaves the E.U., it would remain on the U.N. Security Council, of course, but that’s an increasingly meaningless body. Britain would remain in NATO, but NATO is a military alliance at a time when most of Europe’s security challenges are not strictly military but rather related to economics, policing, even to information policy — all of which are, at least in theory, within the competence of the E.U. And if the E.U. hasn’t come up with solutions, that’s partly because Britain, with this referendum looming, has spent the past several years staying aloof.

“None of your business” is an understandable British reaction to Obama’s visit, in other words, but it misses the point. As the “leave” camp doesn’t seem to understand, we live in an interconnected world, where events in one country necessarily affect those in others. The United States needs Great Britain to stay great, both for their sake and ours.

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