Well the example is there in the next line itself.
US was built by immigrants built with blood and sweat of its own people unlike UK which built its wealth on the blood and sweat of its colonies
US became a superpower by espousing freedom and liberty. UK became a superpower through occupation and suppression of freedom in its colonies. When the sun finally set on the British empire, it became a mere regional power with little or no influence over its former colonies and I certainly will not elevate it to the status of super power along with US. To make the matter worse, when it came to leaving the occupied colonies UK made a mess of things and left with trouble brewing in almost all its former colonies(Ex., Africa and South Asia). The history of Europe makes it difficult for the countries to get together under one umbrella. If you can do it...that would be great but i doubt it. Combine this with the fact that the demography of Europe changing due to ageing population, I do not see Europe anywhere near to being a super power as US.
Let us be frank. The US was once a British colony and attracted immigrants from across Europe (and Asia) travelling to the New World for a new and more prosperous life. Many Europeans shipped out of English ports for the ocean passage.
I dont believe I can describe this New World in a few paragraphs but it did include many immigrants who were little more than an unscrupulous rabble to whom nationhood, law and order, taxes for the benefit of community and restraint in the pursuit of material gain were anathema. Wild West material.
Of course there were also many who wanted to build a God-fearing nation; to get it right second time around.
The former, unscrupulous types were amongst the leaders who created an unholy alliance of the worst, all of whom packaged unwholesome "greed" and "self-interest" as the "American Dream". And they advocated liberty and freedoms to choose and inalienable rights to control their own destiny.
They were led by "Founding Fathers". A clique of smart, greedy un-godly men who had outgrown the constraints of a civilised state, especially one with a silly king. Indeed Bush/Chenney later showed that they had similarly outgrown the moral and legal constrains of a civilsed world.
These Founding Fathers were hypocrites and pious men who considered the New World meant material wealth, nothing more. This mentality remains today within the GOP.
They fought well against the British for their right to be independant. Then they embarked on another war of independance; the bloodiest war in US history, against each other. Apparently the freedom to choose had been rescinded.
The USA stole many "states" including California, New Mexico, Arizona, Texas, Florida, Louisiana etc mostly Indians and Mexicans and perhaps it is poetic justice that the Mexicans have retruned; their own de-facto albeit passive war of independance almost won.

As for US becoming a superpower by espousing freedom and liberty, I would suggest that "superpower" means USA has bigger and/or more bombs, nothing more. The USA is not in Britain's class, never will be. Perhaps this explains its inferiority complex towards the British?
The USA does not practice freedom and liberty as you say and we have clearly seen this over the past 8 years. Your country's unparalleled greed compelled it to illegaly attack Iraq and I often recall the children killed and maimed in the opening salvos alone. Your people deliberately allowed the rabble to sack this cradle of humanity, having no interest in "relics of history"; Old World values?
The USA continues its alliance with Israel, signed and sealed in Hell which has sickened the Old and Third Worlds and you have protected an out-of-control Israel with their policies of state sponsored assassinations, collective responsibility (where they punish families of resistance fighters). You have allowed them to commit a holocaust against a homeless and vulnerable population terrorised for decades. And you all appear to be quite comfortable with this arrangement.
I often marvel at USA's blind faith in its own incompetence. Surely you learnt from Vietnam? We did send military advisers but you did not listen because we were Old World? And the consequence was a humiliating defeat and indecently hasty retreat.
There are many reasons why Britain lost its Empire. The two World Wars, the loss of so much iron from our blood, the cost demanded by USA, the aspiring colonial power....all can not be ignored.
But as I have said, perhaps the saddest thing about USA is its inferiority complex. In many ways the USA is like Bush, the son. He must also be a great disappointment and an embarrasement to his father.