By Kevin Connolly
BBC Washington correspondent
Barack Obama faces tough economic and foreign policy challenges
History will remember Barack Obama for the change he personifies.
As America's first black president he will write a new chapter in a long story that began in slavery and persecution and has not yet ended in equality.
But he is determined that history will remember him as an agent of change, not just as a symbol of it, and that will not be easy.
Mr Obama has been a brilliant candidate in many ways - the muscular poetry of his oratory is matched by his flair for the nuts and bolts of campaign organisation.
But he has been lucky too.
Even the banking crisis, which called into question the competence of Republican economic stewardship, came helpfully at a moment when he and John McCain were neck and neck in the polls.
He has not been lucky though in the circumstances which greet him as he takes office.
Funding the promises
The economy is in recession and the US, at war on two fronts overseas, faces profound questions that will require quick answers.
Mr Obama though will have at least one asset no other American president since Kennedy has enjoyed - a huge reservoir of international goodwill.
That is based partly on the simple fact that he is not George W Bush and partly on the widely-held belief that in picking a black president the United States is somehow closing one of the darker chapters in its own past.
It is not clear of course how deep that reservoir might be nor how long it will last - and it will not help much with the most pressing problem of all, which is what to do about the US economy.
Mr Obama has promised a tax cut to 95% of Americans and plenty of other things that will cost money too - like better access to health care for the 45 million people here without insurance, and an army of new teachers, with improved salaries, for the school system.
None of that will be cheap - and Mr Obama is inheriting a budget deficit running into hundreds of billions a year and a national debt which is about to go above the $11 trillion (£6.9 trillion) mark.
Whether or not Mr Obama is able to keep his campaign promises, he will be drawing heavily on his extraordinary gift for communication - expect that to be one of the hallmarks of his time in office.
He is a gifted speaker and in times of national grief or doubt it is hugely important for Americans to have a president able to capture, shape and occasionally lift the national mood.
Those gifts will be equally important if President Obama finds himself in the depths of recession having to explain why campaign promises are being deferred or even dumped.
Obama has inspired great hope and high expectations in many black voters
How that goes down with the American people will depend on how successfully Mr Obama manages another of his campaign promises - the rather nebulous goal of bringing Americans together.
The new president sees himself as an essentially post-partisan figure and his rhetoric is filled with urgent talk of bringing together a fractured society so that young and old, black and white, rich and poor, and gay and straight all work together with a sense of common purpose.
On the campaign trail, this made Mr Obama seem psychologically interesting - almost as though he were yearning for the US to be a better version of itself. It will be interesting to see how he intends to bring that vision to life in a country where there are still profound racial divisions and which thrives on the vigour of its competitive political process.
Look out for widespread use of the internet in the implementation of the Obama vision, by the way. Mr Obama's campaign was creative in using the web to raise funds and drum up an army of volunteers - he might have something similar in mind for his presidency.
Mr Obama will find himself tested and perhaps defined by foreign policy issues just as his predecessor was.
He has to find an exit strategy for Iraq that does not somehow enhance the regional power status of Iran.
And of course the issue of Iranian nuclear ambition cannot be ignored either. How will President Obama react to pressure from Israel, or from his own military commanders, to bomb Iran's reactor to prevent it from developing a bomb? We might know very soon.
In Afghanistan Mr Obama has talked of putting in more American troops and finishing the fight with al-Qaeda. That is easier said than done and if a beefed-up Afghan campaign goes badly, it will reflect on his judgment and damage his standing.
There remain the challenges of fighting effectively around the Pakistani border without alienating that turbulent ally. And that is before the problems of rebuilding - or rather building - Afghan civil society are contemplated.
Mr Obama has made history by winning power. As he attempts to make history in the way he exercises it, he will be weighed down by high expectations. He is going to need all the many gifts - and all the luck - that got him here.