US future

British, once the biggest creditor in the world before 20th century, became hugely laddened by debt after world war 1.
America, once biggest creditor in the world after world war became largest debtor in human history in 20th century.

However, US now is still very strong compare to British during early period of 1900 when the super power began shifting to US. US economy had then twice the size of British. British was then producing 4 times as many bicycles as the US did, but US manufactured 12 times as many cars. (quoted from "The PostAmerican World").

Some of interesting fact.
US has been maintaining their economy share of 25% since after world war 2.
US defense spending including defense R&D account for almost 50% of global spending, but it is only account for 4% of US GDP.
US still hold the highest shares of international patents in the world of 28%, although Asia is catching up in market share fast.
New technology like nanotechnology will likely be commercialized successfully in US than other part of the world mainly because of the more quality entrepreneur in US and better developed venture capital market that ensure adequate funding. Quality entrepreneurs are created by US unique education system to ours Asia that encourage creative thinking and independent thinking.

US dominates the world economy, dominates the world in science and technological progress, dominates military size. Entrepreneurship that create creativity and innovation remain strong.
For those reason, I think US will remain it's dominance position and leads the world in at least next few decades.