In the final episode of his six-part series entitled Age of Empire, the BBC's Jonathan Marcus examines the challenges which lie ahead for the world's greatest power.

Ellis Island in New York Harbour is like a giant railway station without any trains.

Its echoing halls and dormitories were once the first land-fall for thousands of would-be immigrants into the United States.

Its peak years of operation were between 1892 and 1924, and it is estimated that Ellis Island was the gateway for some 12 million people, whose descendants now make up over 40% of the country's population. The Immigration Centre is now a museum; testament to the attraction of the US and symbol of a particular view of American history and of America's place in the world.

It conjures up the image of a new land cut from the prairies and wilderness by the "huddled masses" of the old world "yearning to breathe free".

This phrase is taken from the famous poem by Emma Lazarus that is inscribed on the plinth of the Statue of Liberty.

She describes "Lady Liberty" as lifting her lamp "beside the golden door"; a view of America that sees it as the land of eternal promise and betterment; a veritable light unto the other nations of the world.

Glory days

This view of the American example is as powerful today as it was then.

Ellis Island is a potent symbol of all that America has stood for

For the final article in this series I went out with the US Border patrol in Laredo, Texas.

Standing on a bluff overlooking the Rio Grande River - the international frontier between the US and Mexico - I could see where people cross virtually every day.

Many are caught.

Others simply drift away into domestic or casual employment, fuelling a huge black economy of paper-less workers, with whom the US and Mexican governments are belatedly trying to deal.

Laredo was an interesting place to end this series.

On the face of it this old cowboy town seems disconnected from the wider world.

But standing in the town square one gets a powerful sense of the tides of empire that have swept over this remote back-water.

Laredo was established in the 18th Century by an officer of the Spanish Colonial Army as one of the northern-most settlements in "New Spain".

It then became part of an independent Mexico and it was finally absorbed into the US after the annexation of Texas by Washington and the Mexican-American war of 1846.

It is quite a history for such an out-of-the way place.

But as if all this was not turbulent enough, in the town square there is a statue of General Ignacio Zaragoza, hero of the Mexicans' efforts to eject French forces from their country in the mid-19th century.

It serves as a reminder that in far-away Paris, Napoleon III, rather bizarrely, had wanted to add Mexico to France's overseas empire.

Other people's money

So the message in Laredo is that empires very much come and go.

But what of the US and its dominant position in the contemporary world?

Kissinger believes the strength of anti-American sentiment is damaging the US

Most commentators I have spoken to believe that, in the short-term at least, US power will actually grow.

But economist Will Hutton told me of what he calls "the dark underside of the United States economy" - the condition of its international accounts.

Put bluntly it owes the rest of the world $3trn dollars, and that is debt which is increasing by $500bn a year.

Mr Hutton makes a persuasive case that America's current strength is built on other peoples' money and he fears that the US economic bubble will eventually burst.

"People simply aren't going to carry on lending America billions and billions of dollars and America's not going to carry on borrowing it," he told me.

In his view America's economic strength is "built on sand".

Power and mistrust

For now, though, US power seems unchallenged.

And for all the talk about foreign policy and the forthcoming presidential elections, a tour of the main think-tanks in Washington DC led me to believe that the differences between the two major parties were matters of degree rather than substance.

For all of America's abiding attractions though, I was left with a powerful sense of the hostility it evokes.

Veteran diplomat Henry Kissinger told me that anti-Americanism was not new, but he said he was deeply troubled by the extent and intensity of the contemporary phenomenon.

This, he feared, was leading to a decline in America's legitimacy.

Commentator Fareed Zakaria shared some of these concerns.

His worry was that if things continue as they are, then America could end up being, as he put it, "not only the most powerful country in the world but also the most distrusted country in the world".

That is a warning that whoever wins the November election might do well to heed.