If you lack a taste for flags and incessant displays of patriotism, the US is not the place for you.

What is taken for granted by its citizens can sometimes amaze the visitor. At times it can seem that the Stars and Stripes is everywhere, hoist over every backyard and public building. Oaths and declarations and affirmations of American exceptionalism are unending. Few of the natives think it odd. Most believe every last word of their lullabies.

This is, of course, fair enough. The United States fought hard for its independence. It survived through a civil war whose cost is still too little understood in Europe. Through thick and thin the republic has held itself together by an adherence to a constitution without parallel in history. That, too, has been a struggle.

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Numerous presidents have put the constitution at risk. A laughably corrupt duopoly, a confederation of oligarchs and corporate money, mocks its principles daily. Sometimes, as now, the American system creaks and groans. To young black men facing a parody of justice, or to Occupy Wall Street demonstrators, or to those whose children die thanks to gun mania, the country's resemblance to a democracy can seem faint. But most still believe in it fiercely.

There was proof enough of that after 9/11. For a few days, George W Bush had a nation and half the planet at his back. Then he blew it. There are tangled reasons for the extraordinary error, most of them to do with the nature of a man and his backers, but one part of the transformation had something to do with America's understanding of itself.

"With us or against us": that was blunt enough. "We" and "us" then became liberty itself. Thanks to that constitution, born of the Scottish Enlightenment, the United States claimed copyright both on moral law and on the right to pass moral judgement, with punitive sanctions, on all the inhabitants of a planet. The Twin Towers justified everything, good or criminally bad.

One tiny, trivial consequence is that I never expect to see America again. Unless this line of work makes it unavoidable, I don't expect to see again the hills of New Hampshire or the deserts of Arizona. Faced with the demands of a homeland security apparatus or the data-harvesting machine, I won't have another glimpse of the back lanes of Greenwich Village, or the hideous Santa Ana Freeway. I won't be interrogated just to keep files up to date in the new America.

This was not the early promise. When Barack Obama gave his speech in Normandy yesterday, he invoked - for his domestic audience, at least - a vision of bands of brothers storming ashore, far from home, to bring the gift of freedom to an imprisoned Europe. The squaddies who fought through North Africa might have demurred over dates, but the president's words were powerful. They asserted again that it is the purpose of the US to do good in the world.

I understand the tradition. It is of a piece with Woodrow Wilson's determination to impose himself on the Versailles conference and create the first of those new world orders. It fits with Franklin Roosevelt's belief that the British Empire should not survive the Second World War. It also follows the tradition of Harry Truman deciding that a post-war Britain would be better off bankrupt, or Eisenhower humiliating Anthony Eden over Suez.

The nonsense of the special relationship should be well understood by now. It existed, briefly, as a war expedient, with a tinge of Anglo-Saxon racism, and it has never once caused the US to lose sight of its national interests. Each act has been driven by the conviction - one that would have been understood in Victoria's Britain - that the empire always knows best. What is good for America, what seems proper and democratic to America, is good for all.

As a child, seeing those sleek, black, shark-like boats coming in and out of the Holy Loch, the proposition began, bit by bit, to fall apart. The puerile accusations of "anti-Americanism" contain the same dissonant notes to this day. I'm not sure I am capable of being an enemy of the country of Dylan and Charlie Parker, Edward Hopper and Robert Mitchum, Thomas Jefferson and that constitution. But I'm pretty sure I can remind an American president of what self-determination means in his tradition.

Granted, Mr Obama didn't say much when he offered up a few quotes in Brussels. It was fun, in fact, to see all the broadcast media omit his qualifying remark that any decision in September is - for who'd have thought? - up to the Scottish people. That the British Government's urgent diplomatic efforts got an endorsement from the American rather than Vladimir Putin must count as a windfall in Whitehall. But still, has the man who has FDR's office forgotten the meaning of self-determination?

Mr Obama knows better. His father suffered greatly at Britain's hands for trying to win Kenya's independence. The president has taken a kind of ironic and satirical pride in the fact. This leader of the free world has meanwhile congratulated Ireland on its liberty. Who, the Queen included, does not these days give the Irish their state visit and their due?

So the President said this: "We obviously have a deep interest in making sure that one of the closest allies we will ever have remains a strong, robust, united and effective partner". No President of the US would say otherwise. But the dissemination of liberty and democracy, the contest from which America was born, the ideals that give the country noisy pride, do not depend on where nuclear boats are parked.

The UK's force de frappe is of no consequence to the US in any case. The periodic appearance of dutiful allies, a camouflage of "international opinion", matters more in domestic propaganda wars than in real conflict. Britain is "strategically important"? Where, exactly, and in which circumstances? The fear of upsetting the controllers of the dollar economy matters more in British politics, meanwhile, than any vote at the ballot box.

But that's OK, as a debating point. As an improbable romance, Scotland's vote in September might even remind Americans of their founding principles. Democrats might recall FDR the anti-colonialist. On the other hand, a few words on what it means to be independent generally do the trick among Americans who know their constitution.

In 1775, the Minutemen of Culpeper, Virginia, got a flag. To this day, Americans respond to what was stitched on the banner. It said: "Don't Tread On Me". There are numerous ironies in having a small and mostly blameless country trodden on lightly by a Democrat president, but as Scots helped make the best and worst of Obama's America, we'll manage.

When I first saw the sleek boats in Holy Loch, Scotland's relationship with the US was clear enough. By every measure of opinion down the decades, we didn't want those things, but there they were. An imperial democracy, like a colonised democracy, is a contradiction in terms. Mr President forgot.