Barack Hussein Obama was today sworn in as 44th president of the United States of America in front of quite possibly the largest mass of humanity ever to have gathered in one place for a single political moment.
As many as 2 million people in Washington's National Mall heard their new commander-in-chief deliver a sombre 20-minute speech in which he acknowledged that the country was in the midst of crisis – mired in wars, its economy struggling and its national confidence sapped. He promised the largely silent crowd that the challenges would be met, but warned it would take time, some sacrifice, a new form of politics and a re-engagement with the world, in which America would recognise that "power alone cannot protect us, nor does it entitle us to do as we please".
President Obama took the oath just after midday under a crisp and cloudless azure sky in front of the glistening cream dome of the Capitol, which, it is now accepted, was partly built by slaves.
The day, cold enough to freeze breath, had begun with millions of individual journeys by coach, train and on foot as the crowds began converging before dawn for a moment widely taken as one of renewal and of double foreclosure. This was to be the end of the last eight years of Republican rule and of the bars which, at any previous time in history, would have made the election of an African American president unthinkable.
They had come to celebrate – and for days they had been doing just that in parties and balls all over town. The cheer as Obama swore his oath on Lincoln's Bible rippled and roared all the way from the Lincoln Memorial to the Capitol, nearly two miles away.
But when Obama spoke it was immediately apparent that the tone of this inauguration was grave, addressed as much to the hundreds of millions tuned in around the world as to the shimmering sea of upturned faces in front of him.
"That we are in the midst of crisis is now well understood," he said. "Our nation is at war, against a far-reaching network of violence and hatred. Our economy is badly weakened, a consequence of greed and irresponsibility on the part of some, but also our collective failure to make hard choices and prepare the nation for a new age.
"Homes have been lost; jobs shed; businesses shuttered. Our healthcare is too costly; our schools fail too many; and each day brings further evidence that the ways we use energy strengthen our adversaries and threaten our planet."
If this was read as a repudiation of the previous eight years of Bush, there was plenty more of it. There was, said Obama, a nagging fear that American decline was inevitable; he wanted an end to "petty grievances and false promises"; the time had passed for "protecting narrow interests and putting off unpleasant decisions"; a nation could not prosper long "when it favours only the prosperous".
In one of the few lines to be greeted by fervent applause, he turned to defence, proclaiming "we reject as false the choice between our safety and our ideals". In a thinly-veiled reference to Guantánamo and torture he promised not to abandon the rule of law and human rights "for expedience's sake".
There was further implicit criticism of his predecessor's policies in his comments on science and the environment. He vowed to "restore science to its rightful place" and made several references to climate change, acknowledging the threat to our planet and saying America would in future "harness the sun and the winds and the soil" for energy.
On international affairs, he singled out the Muslim world, offering "a new way forward based on mutual interest and mutual respect. America would leave Iraq "to its people" and forge a "hard-earned peace" in Afghanistan.
All inauguration ceremonies consciously celebrate, and reference, both the constitution and former presidents.
Four ghosts hovered over yesterday's ceremony. Lincoln's Gettysburg address gave the new president the overarching theme for his speech – the "new birth of freedom". It is Lincoln, the gangly lawyer from Illinois, who has fascinated Obama more than any other previous president.
Lincoln's Emancipation Proclamation freeing slaves and yesterday's inauguration were in some ways bookends to the darkest stain on America's history. For many in the crowd this was the over-riding reason for the pilgrimage to Washington. Obama put it simply: "A man whose father less than 60 years ago might not have been served at a local restaurant can now stand before you to take a most sacred oath."
The second ghost was Martin Luther King, who, had he lived, would have been an 80-year-old spectator. No one in the crowd could have been oblivious to the echo across time of the words that had rung out from the other end of the National Mall 45 years previously.
And then there were JFK and FDR. Obama's call for responsibility and sacrifice recalled both Kennedy in 1961 and Roosevelt's heartfelt cry in 1933: "We now realise as we have never realised before our interdependence on each other."
Obama's serious tone and his unflinching acknowledgement of the economic hurricane blowing through America echoed Roosevelt's speech at the time of the last serious global depression, in which an incoming president vowed "to speak the truth, the whole truth, frankly and boldly … with a candour … which the present situation of our nation impels".
The endless crowd listened solemnly to the same sentiments today. They might have come wishing for something more uplifting, but, for many, the day reached beyond symbolism to a moment of genuine transformation after which nothing could be the same again.
As Obama headed back into the Capitol building at the end of the ceremony clouds began rolling over what had until then been a pure blue sky. But there was one final, rousing cheer as the helicopter carrying George W Bush rose over the gleaming dome of government and took the former president off to Texas – and out of public life for ever.