The decline of US power?
Standing on the Washington Mall at the turn of the new millennium, it was impossible not to be struck by America's power and global pre-eminence.
Victory in the Cold War made it the hegemon in a unipolar world.
Few argued when the 20th Century was dubbed the "American Century", a term first coined in the early 1940s when the country was still overcoming its isolationist instincts.
Even the New Year's fireworks, which illuminated the obelisk of the Washington Monument in a way that made it resemble a giant number one, projected the country's supremacy as the world's sole superpower.
Over the past 15 years, America's fortunes have changed with dizzying speed.
First came the tremors: the dot-com bust and a disputed presidential election in 2000. Then came the massive convulsions: the destruction of the Twin Towers in 2001 and the collapse of Lehman Brothers in 2008.
Long wars in Afghanistan and Iraq have exacted an enormous blood price - the lives of 6,852 American military personnel - not to mention immense financial expense, estimated to be as high as $6 trillion (£3.9tn).
The detention centre at Guantanamo Bay has undermined American ideals, just as the NSA and Wikileaks spying scandals have undercut American diplomacy.
George W Bush, a president with a Manichean worldview, was widely seen as over-eager to project America's military might, without adequately considering the long-term consequences.
Barack Obama, who campaigned in 2008 on a platform of extricating America from its unpopular and exhausting wars, has drawn criticism for disengaging too much.
Under both presidents - the first an impulsive unilateralist, the second an instinctive multilateralist content sometimes to lead from behind - America's global standing has been diminished.
Lost fear factor
Polls regularly show that Americans recognise that their country's international standing has waned.
Among the young, this trendline has fallen sharply. Only 15% of 18-29-year-olds believe that America is the "greatest country in the world", according to Pew, down from 27% in 2011.
Tellingly, however, there has been no great public outcry.
No longer is there much appetite for America playing its long-standing role of global policeman, even in the face of the rise of the group calling itself Islamic State.
The cost, human and financial, is considered too great. Americans increasingly think that other countries should share the burden.
Obama, while continuing to trumpet "American exceptionalism", regularly prefaces remarks on foreign affairs by acknowledging the limits of US power, again with little public outcry.
The upshot is that the United States is no longer so keen to exert leadership in an increasingly messy world.
Yet one of the reasons why the world has become so disorderly is because America is no longer so active in imposing order.
Over the course of this century Washington has lost its fear factor.
Ignoring the White House
World leaders nowadays seem prepared to provoke the wrath of the White House, confident that it will never rain down on them.
It explains why the Syrian President Bashar al-Assad, after unleashing chemical weapons against his people, continues to bombard them with barrel bombs.
Why Vladimir Putin annexed Crimea, and also offered a safe haven for the NSA whistleblower Edward Snowden.
And also why Benjamin Netanyahu thumbed his nose at the Obama administration, by accepting an invitation from the Republican congressional leadership to address a joint session of Congress, a platform he used to lambast the Iran nuclear deal.
Assad's flouting of American warnings is especially noteworthy.
In killing so many civilians with chemical weapons, he flagrantly crossed the "red line" imposed by Obama, but escaped punishment.
The president was unwilling to carry through on an explicit threat, in what was the biggest foreign policy climbdown of his presidency and also one of the most significant in the past 50 years.
Even supporters of Barack Obama believe he made a fatal strategic mistake, because it demonstrated endless flexibility and a lack of American resolve.
Needless to say, despots around the world took note.
America's reluctance to launch new military actions has also had a major bearing on the nuclear negotiations with Iran.
Tehran has managed to extract notable concessions, such as the ongoing ability to enrich uranium, hitherto ruled out by the Americans.
It has played a weak hand strongly, because it knows that America has what the foreign affairs columnist Thomas Friedman calls "an empty holster".
Nor is it just America's enemies who no longer fear the White House to the extent they once did.
In recent months, two close allies, Britain and Australia, have defied the Obama administration by joining the Asian Infrastructure Investment Bank.
By signing up to the AIIB, they are effectively endorsing Beijing's effort to establish financial rivals to the Bretton Woods institutions, the World Bank and International Monetary Fund (IMF), which are dominated by America.
By seeking improved commercial and diplomatic relations with China, Britain and Australia are also hedging.
They suspect that America will not be the dominant Pacific military power indefinitely, nor the world's foremost economic powerhouse.
Other American allies would complain that the "dependability factor" has also gone.
Israel feels badly let down by the Obama administration over the Iran deal, and relations between Benjamin Netanyahu and Barack Obama are poisonous.
The president, by using deliberately ambiguous language, has even signalled that his administration might end its traditional protection of Israel at the United Nations.
Like Israel, Saudi Arabia has been enraged by the prospective nuclear deal with the Iranians.
Riyadh also knows that America is no longer so dependent on its oil, the cornerstone of the relationship since the end of World War Two.
Egypt was angered in 2012 when Obama said Cairo was neither an ally nor an enemy.
Later, the State Department issued an embarrassing correction, and reinstated Cairo as a "major non-Nato ally."
Maybe Obama's Egyptian error, and the slight it conveyed, was truly a Freudian slip.
After all, he hasn't invested the same energy nurturing alliances as his predecessors. The detached air that has been a hallmark of his presidency also extends to foreign affairs.
Here, I gather, Obama recognises intellectually that he could do far more in terms of massaging the egos of world leaders, but cannot quite bring himself to do so.
Indeed, a common complaint is that the Obama administration has prioritised normalising relations with its one-time enemies, Iran and Cuba, at the expense of fostering longstanding friendships.
Realising that America is no longer so supportive, and no longer so engaged in the Middle East, the Saudis have recently taken military action of their own in Yemen.
There's also been a warming of relations between Riyadh and Moscow.
And Egypt launched airstrikes in February against the Islamic State group in Libya.
America's standing in the Middle East has unquestionably waned, along with its ability to shape events.
More surprising has been its slippage in Africa, Obama's ancestral home, and Asia, the focus of his much vaunted pivot.
In Asia, America's median approval rating in 2014, as measured by Gallup, was 39%, a 6% drop since 2011.
In Africa, the median approval went down to 59%, the lowest since polling began, despite Obama hosting the US-Africa Leaders' Summit in Washington in August, last year.
It even dropped in Kenya, his father's birthplace.
America's diplomacy has also been complicated by the dysfunction and hyper-partisanship in Washington.
Republican lawmakers actively sought to derail the Iran nuclear deal by sending a letter to the supreme leader Ayatollah Ali Khamenei.
President or Congress?
House speaker John Boehner invited Netanyahu to address Congress, knowing it would infuriate the White House.
Democrats with reservations about free trade have tried to sabotage the Trans-Pacific Partnership, the biggest trade deal since Nafta.
There's also been strong congressional opposition to one of the big plays of Obama's second term, the rapprochement with Cuba.
Should countries listen to the president or Congress?
America cannot even lay claim any more to its great, uncontested boast since 1872, of being the world's largest economy.
The IMF now estimates that China's economy is fractionally bigger.
Yet it would be a mistake to exaggerate the downsizing of American influence.
US military spending continues to dwarf its rivals, and up until last year amounted to more than the next 10 countries combined.
In 2014, America spent $731bn, compared to China's $143bn.
Even though China's economy is now larger, America's per capita spending power is in a different league - $53,000 to $11,868.
Though America is contending with the rise of the rest - China, India, Brazil, Germany and Russia - it has not yet been overtaken by emergent rivals.
Indeed, there are foreign policy thinkers here who predict that America will preserve its pre-eminence for at least another 20 years.
Yet the unipolar moment ushered in by the fall of the Berlin Wall has proved to be just that: momentary.
Moreover, hopes of a new world order following the collapse of the Soviet Union have given way to widespread pessimism about the spread, even the contagion, of global disorder.
Gone are the certainties of America's Cold War thinking, when the containment of communism governed its international actions.
Gone are the doctrines that gave US foreign policy such a rigid frame, throughout the Cold War and in the aftermath of 9/11.
Gone, too, is the notion that every fight is an American fight and along with it a redefinition of what constitutes the US national interest.
Barack Obama has instead advocated pragmatism and diplomatic dexterity, trying to steer a path between America being overextended and undercommitted.
Maybe the overriding challenge for US diplomacy over the next 20 years is to strike the proper balance.