A couple of days after 9/11 I happened to be in London, stranded on my way back to Washington from a spectacularly ill-timed trip to Asia. As I walked from my hotel that morning, still struggling, like everyone, to take in the full dimensions of what had happened, I noticed a blizzard of posters.

On almost every street lamp and available piece of wall were hastily printed sheets of A4 paper, bearing in bold lettering a simple request: “Please observe three minutes’ silence at 12 noon today in memory of those who lost their lives in the awful events in New York and Washington on Tuesday.”

I have no idea to this day who had posted the flyers — hundreds of them, all along the South Bank. To a cynic, I suppose, it might have been a devilishly clever ploy by the CIA to build support quickly for the US in world opinion. Or perhaps it was some desolate band of American tourists or City workers seeking to elicit sympathy from friendly hosts. But it looked much more like the simple, hurried work of ordinary Londoners, spontaneous and genuine.

That someone should go to the trouble of typing such a plea, printing all those flyers, then walking through Central London sticking them to lampposts and walls, struck me then as a remarkable gesture of human empathy, as poignant and profound in its way as Le Monde’s “We are all Americans now” headline or Buckingham Palace’s decision to have the band of the Household Cavalry play The Star-Spangled Banner.

Indeed, the flimsy bills were actually more heart-rending than those more famous expressions of condolence. This was not the work of a newspaper editor with an eye to posterity or some government official with a felicitous PR touch, but the ingenuous response of a distressed citizen to the unimaginable heartbreak that had unfolded over the previous 48 hours.

The intense little gesture captured, in fact, the mood in most of Britain, Europe and what we like to call the civilised world that September day five years ago.

For all its geopolitical ramifications, 9/11 evoked principally, at the time, the rawest of emotions at a fellow human’s plight. What eyes didn’t weep at the sight of those wretched figures leaping to their deaths? What stomach didn’t churn at the thought of those helpless pilots as their throats were slit in their cabins? What heart didn’t lift a little at the story of the desperate passengers on United Flight 93, fighting back against the monstrous aliens violently wresting control of their doomed lives?

The rest of the world has always had a complex set of attitudes towards America — a mixture of envy, admiration, disdain, gratitude, exasperation, hope and, sometimes, fear. But that day, that week, America evoked only the sort of strenuous affection that causes a complete stranger to go out and stick bills on lampposts.

But that instantaneous solidarity with a stricken superpower was not, as it turned out, anything like a good predictor of the history that would unfold over the next half a decade.

As it prepares to commemorate the fifth anniversary of the attacks, America stands reviled in the world as never before. It is a remarkable turnabout. In the same amount of time that elapsed between the assassination of Archduke Ferdinand and the Treaty of Versailles, in as many months as passed between Germany’s invasion of Poland and D-Day, the US has gone from innocent victim of unimaginable villainy to principal perpetrator of global suffering.

So complete has been this transformation in global sentiment that it is inconceivable now, should America be attacked again, today, that the tragedy would elicit the same response. There would be horror and sympathy in good measure, certainly, from most decent people. But there would also be much Schadenfreude, and even from the sympathetic a grim, unsmiling sense that America had reaped what it had sown.

The facts — the historical events — that have brought about this changed perception of America are not in dispute. They can be tracked chronologically, almost from the moment the twin towers came down.

Sympathy for a grieving America translated quickly into general support for the US war against the Taleban. But within a few weeks that support began to drain, as civilian casualties mounted and some questioned whether the US was doing enough to address the “root causes” of terrorism, in particular the Israel-Palestine conflict.