Why Washington is worried about Brexit

US President Barack Obama gestures as he gives a speech in central London Image copyright Getty Images
Image caption Mr Obama has urged Britain to stay in the European Union

Before plunging into the question of why America cares so much about whether Britain remains a member of the European Union, let's make a quick distinction.

America doesn't. The political establishment does.

I suspect if you stop the average American on the street and ask what they think about Brexit, the assumption would be that you were talking about some delicious new biscuit.

Brexit, the snack you can eat between meals without ruining your appetite. Or a Brexit a day helps you work, rest and play. Or something like that…

Image caption Brexit is not a morning snack

Outside of the posh papers like the Washington Post, the Wall Street Journal and the New York Times there has been next to no discussion of next month's referendum in the UK.

And I am willing to be corrected, but I think there has been precisely zero discussion of this issue on the US TV networks - with the notable exception of when Barack Obama was in London and weighed in at his news conference with David Cameron.

And even then, for the US audience Britain's membership of the EU was of far less consequence than the lunch the president and the first lady had with the Queen and Duke of Edinburgh the day after her 90th birthday - and, of course, the picture of the trip: the president and the prince in his dressing gown before bedtime.

So having made that distinction, let's ask this question: does the Washington establishment really care that much? The simple answer is yes.

Image copyright Getty Images
Image caption For the US, Britain is often seen as the bridge to Europe

First, let's deal with a canard. When I was in London with the president on his recent trip, it was suggested to me that the "establishment" in the US weren't really that interested one way or the other in the whole Brexit debate, but were intervening at Cameron's behest. He needed some help; they lent an old ally a hand.

But this is arrant nonsense. You don't get a whole pile of former treasury secretaries to sign a letter to The Times of London if they don't believe what they're saying. The same goes for former US secretaries of state, defence secretaries, heads of the CIA and national security advisers.

While it is certainly true that looking at the polls Cameron clearly does need a hand, and they have lent an old ally a hand, don't muddle cause and effect. This might be the effect of their intervention, but the reason for sticking their giant stars and stripes oar in is that there is profound disquiet in Washington about what the effects of a Brexit could be. And the concerns are NOT just about what harm this might do little old Britain (although that is a part of it); it's about the wider geopolitical ramifications that might flow from it.

Dealing with Britain first, the US has always believed that the UK has been a vital part of the Transatlantic partnership. At times when relations have been fraught between Europe and the US, you will hear US policymakers talk about how the UK has acted as a bridge - able to relay messages to Brussels that Washington is anxious to convey, and vice versa. The "special relationship" means that there is unprecedented intelligence sharing and defence cooperation. There is something like 800 British service personnel in America at the moment, serving in all the branches of the US military.

Image copyright Getty Images
Image caption Mr Obama said a trade deal between the US and the UK is not guaranteed

On trade, the Americans think that Britain will be seriously disadvantaged if it finds itself alone in the world having to negotiate a trade treaty with the US. As the president said - controversially - when he was in London, Britain would be at the back of the queue for any future deal.

And then there is the wider concern. There is serious concern that Britain leaving the EU could destabilise the rest of the European Union. Would other European nations start to demand their own referendums on remaining part of the EU? Could the whole European project unravel? What would be the effects on the global economy at a time when nations are still emerging from the rubble of the 2008 financial crisis? What would be the effect on collective security? Why is Russia so keen on Brexit?

In a world full of uncertainties and known unknowns, and unknown unknowns (as Donald Rumsfeld might say - interestingly one person who was NOT a signatory to this letter), Europe has been a source of stability, and a huge trading partner to the US. The TTIP trade deal is the prize that European and US policymakers are after. Not new sources of instability. Madeline Albright, who WAS a signatory to the letter is absolutely clear: It is in America's and the world's interests that Britain remains inside the European Union. And it is worth noting that there is barely a US politician who has held public office who IS an advocate of Brexit.

So, yes, the American foreign-policy establishment is watching with serious concern and interest. Part of that can be judged by the letters to The London Times, and part of it can be judged by the number of invitations I am receiving to debates in Washington on this subject. It is the plat du jour.

Except in Washington, Brexit is the snack they want to see removed from the menu.