Archimedes said: 'Give me a place on which to stand and lever long enough, and I will move the world'. British foreign policy should be about maximising and exploiting the levers we possess – whether through Europe, the transatlantic relationship or the Commonwealth – not breaking them or throwing them away. 

I have yet to meet any significant western political figure from beyond our shores who can understand why Britain would even contemplate leaving the European Union, which is now a key point of leverage for this country in the modern world. 

In Washington, Tokyo, Beijing, New Delhi or Moscow, let alone in all other EU national capitals, it seems obvious that the UK needs the Union as the platform and vehicle by which to influence events and policy in many spheres. Nowadays, with the possible exception of Germany, a country such as Britain, boasting about one per cent of the world’s population and three per cent of the world’s GDP, is unlikely to be able to hold anything like the position of power to which we continue to aspire, unless this is firmly anchored in a strong alliances and, ideally, a credible regional framework. With the decline of NATO, the only such framework available, unless we seek to join the United States, is basically the European Union.

The Americans have always wanted Britain to play a leadership role in a united Europe – from the early 1950s through to today. It has been a constant of US foreign policy that any 'special relationship' is not based on nostalgia or some mystical solidarity among the 'English-speaking peoples', but on a realpolitik assessment of our capacity to help shape our continent in a modern, outward-looking direction.

Half a century ago, in making Britain’s first application, Harold Macmillan understood this very well. He wrote: 'If we remain outside the European Community, it seems to me inevitable that the realities of power would compel our American friends to attach increasing weight to the views and interests of the Six in Europe, with others who may join them, and to pay less attention to our own. We would find the United States and the community concerting policy together on major issues, with much less incentive than now to secure our agreement or even consult our opinion. To lose influence both in Europe and Washington, as this must mean, would seriously undermine our international position and hence, one must add, our usefulness to the Commonwealth.'

Every one of Macmillan’s words remains as true and powerful today as in 1962 – except that, first, the Six are now the 27; and second, Britain is a much lesser force in world affairs, making the problem he describes more acute.

Last week, President Obama called Britain’s membership an 'expression of the UK’s influence and role in the world'. Leaving the Union would, by contrast, in my view, be a tragic expression of our shrinking influence and role in the world – and the humbling of our ambitions, already sorely tested by the current crisis, to remain a serious political or economic player on the global stage.

Earlier this year, Obama made it clear that America wants 'a strong United Kingdom in a strong European Union', not a weak or isolated UK outside a broken-backed EU. Ironically, his words echoed the 'Strong Britain in a Strong Europe' manifesto slogan on which the Conservatives fought the 1994 European elections. Almost two decades later, the Conservative party now needs a US president to tell it what it once had the confidence to proclaim as common sense itself.