Last week I was working in New York and with time to kill at midday I walked the High Line, a mile-long park created along a derelict stretch of elevated railway built in the 1930s to run freight along Manhattan’s West Side. Originally the trains ran at ground level, with cow boys hired to ride ahead of the locomotive waving red flags, but still pedestrians were killed and the city fathers’ ordered that the railway be raised above ground.

Silver birch and grasses have been planted between the rusty tracks, and green lawns laid in the sidings. There are viewing spurs where strollers rest on wooden decking and look 30ft down upon buses and cars and trucks moving across the island. The Line snakes between the ochre-coloured cliffs of empty factories and depots providing an elevated platform on which folk can fleetingly untangle themselves from reality and watch it happening to other people.

The day was very hot and the Line was busy with lunchtime traffic, groups of students and tourists. Together we drifted into sunlight and through tunnels of tree shade, the leaf shadows marking each of us almost identically. Where I wondered, were we all going, along this revamped railway line which had no purpose save facilitating pleasure and strolling? A European tourist strode quickly past handling substantial camera equipment, as if he had spotted that something was on fire. On a lawn, girls made somersaults. A message, partly obscured by a birch, was painted on the side of an adjacent building: “HONEY, I TWISTED THROUGH MORE DAMN TRAFFIC TODAY”. I had a fleeting image of the world’s population, all seven billion of us, similarly dressed and sharing motivations for satisfying work, happy family life and tasty food, walking together in an enormous crocodile column towards one destination. It was not an unpleasant image, though it was tiring to think of so many of us, and an anticlimax to think we might all turn out so similar; the prospect of more TRAFFIC was tiresome.

In the distance I could see the Statue of Liberty: small, lonely, presumably sun-heated to an unbearable extent, guarding the approaches to the city. Beyond it Staten Island and the shining Atlantic rolling away towards home. The Statue was familiar and stirring in a way very few symbols anywhere in the world are familiar and stirring – I remembered the September morning when Liberty had raised her torch against a Manhattan sky line which emitted lines of white smoke, and then disappeared behind an avalanche of smoke and dust.

I bought an ice lolly from a Mexican lady standing behind a zinc kiosk, its surface so hot she wore gloves to lift the lid, and sat down in the shade of a tall building. Nearby a man with a saxophone sat playing melodies and practicing his scales. A dance quartet, led by a tall girl in khaki hot pants stepped forwards and backwards in unison. The atmosphere in the shade was languid, sloppy, loving… yet also striving.

America’s restless success at business and pleasure maintains a hold on the world’s imagination, and a magnetic attraction for migrants. “Give me your tired, your poor, Your huddled masses yearning to breathe free” offers Liberty. Every immigrant who takes the Oath of Allegiance endorses American leadership, her exceptionalism.

A narrative of discovery and settlement (acts of courage and resourcefulness which also shoved aside the indigenous population) defines America, and her gaze remains forwards, towards the next horizon. In contrast, the United Kingdom still looks backwards, over centuries, along a line of monarchs, a civil war, a civil revolution, English domination, an act of union, industrialization, democratisation, international expansion and plunder and withdrawal and guilt and today a gnawing uncertainty. Our story is organic, multi-layered, extraordinary. Migration is not at its centre, instead we perceive our victories as home-grown, the plucky island people pitted against a continent. Over recent decades we have grown wealthier and easier, but nonetheless felt our authority and confidence seep away. What, we wonder, is the British mission? Which way?

For many, today’s immigration levels feel too high for a crowded island, and add to this feeling of anxiety and lack of control. Yet it is our decency, the fairytales and poems, our nurses and British genius, the old and good institutions as well as the hard-hearted, fragile booming money song which draw the ambitious and determined from Africa, Asia and Europe. These migrants believe in the promise of the UK – our potential to thrive, to adapt and even to lead. They see this more clearly than many of the British do; we are too busy turning in eddies of disappointment. We eddy and we grow dizzy and there is no time, nor the stillness, to script a British storyline for the 21st century.

I reached the end of the High Line and caught a taxi south along the edge of Greenwich Village. It dropped me outside Goldman Sachs’ sand-coloured headquarters. On the quayside yachts registered to the Newport Yacht Club, Rhode Island, were being loaded for the week-end. I stood under the burning sun waiting to cross West Street into the World Trade Centre Memorial Plaza: a grid of swamp white oaks, the two reflecting pools each an acre in size, sheets of falling water separating into silver drops, tumbling down into darkness. Overhead, the replacement towers, around me tourist groups and their guides, veterans of 9/11, each telling the same story in another way. The scent of chlorine; a white rose and a white carnation placed against two of the 2,983 names remembered here, marking their birthdays.

America is also defined by her story of resistance and armed struggle. This begins in 1775 with her insurrection against the British, and builds into the late 20th century and the collapse of the USSR, leaving America the victor of three successive global conflicts, and the world’s pre-eminent military power. But, as we know, the story does not end; it just gets bloodier and more confused and day-to-day. Since the al-Qaeda attacks of 2001, America has faced an anti-imperial insurrection of her own, an existential threat which she calls ‘terrorism’ which will never stop hating American ‘freedom’ and power, her “languid, sloppy, loving… yet also striving”. America’s failure in Iraq and Afghanistan proves just how difficult it is to defeat this enemy, but America is engaged and she won’t stop trying; this is her burden to carry and it defines her.

Britain lost 67 citizens on 9/11 and she has stood alongside America since, and paid her price. She too has been targeted at home by al-Qaeda. But the struggle that truly defines modern Britain ended in 1945, with the defeat of Nazi Germany. Today Germany is one of Britain’s closest allies, whose goodwill is vital if we are to reform the European Union, the post-war customs union which has grown beyond all imagining, integrating Britain and 27 other European states. Is EU reform possible? Would ‘Brexit’ actually change very much? Does anyone have a sensible plan? The answer to the last question is that no one does. Since WWII Britain has maintained the habits and mindset of a global power, without the means to act like one. Successive prime ministers have chosen not to question the notion of British exceptionalism, rooted in our overlapping history with the US, Europe and the Commonwealth, and our victory in WWII. We mean to do well, and can, there have been moments of triumph, but the trend has been decline and in 2014 our relative importance to and influence over the US, Europe and the Commonwealth is diminished. Our post-imperial glide path is over, and we now need to make difficult choices which should have been made decades ago. We need a strategy. A new story line.

How complicated it is to be us at this moment in time, stranded between our past and the future. How American to live in the black and the white. “It looked like the last day on earth” a 9/11 guide was telling his tour group. I stood there, eavesdropping, peering into the dark waters of a reflecting pool.