To be specific I mean both militarily and economically. I know may people will straight away point out countries like China or some can also point out EU, but what I really want to know is HOW and CAN they, or is there any other bloc other than them which can in the real world challenge America?
As with much in life, the first matter to explore is who has the desire for global power? This then comes back to the human wants and desires which are then collectively expressed by what can broadly be described by the term " culture". Most cultures (political and religious) are inherently competitive.
So how will the US loose its power? By repeating the example of the Roman empire - by becoming fat, lazy and the decay of moral standards.
What can be said about a society where the most visible icons are vacuous "celebrities" who parade their promiscous sex and drug lifestyles and the masses actually admire them!! It wasn't always so. America became great while it still had moral and ethical belief as its guiding light.
So the question then can be asked - which cultures exhibit the desire to challenge the prevailing world order?
China is definitely one of my picks. In recent yrs China has made it very plain that it no longer wishes to be inward-looking but wants to become the dominant power on the planet. As it happens, western liberal politics is actually enabling that to happen as we speak by rampant increasing trade, investment and exporting jobs (ie wealth) from the west to China. Can China supplant the US? My pick is absolutely and given China's history and cultural outlook on the rest of the world, it would not be a benign power that upheld freedom but a highly controlling one ( such as in Orwell's book "1984"). Consider also the horrors of the "Cultural Revolution". (read book "Wild Swans" by Jung Chang)
Seeing the growing weakness and decay of the US, China is currently spending up large on military capability that goes way beyond any reasonable defense needs. This already concerns the US and in 5-10yrs could match or even surpass.
Given I've chosen to focus on cultural competition, the other megatrend in my view is that of Islamization (or Islamo-facism as some have described it). The muslim world view is very disturbing to a western liberal such as myself (read the koran yourself - see link in sources)and the recent aggressive action against western values and societies (from 9/11, to French riots, to Sydney riots etc) is just the tip of the iceberg. With western national identities and cultures being ever more diluted in favor of "multicultural societies" (which include massive Muslim enclaves - in Sydney the Police fear to enter such neighborhoods), the collective will of these multi-cultural societies to stand against completion from other cultures has been substantially weakened and continues to erode from the inside. You need look no further than the impotence of the French to deal with the riots decisively.
Muslim nations have chosen non-conventional warfare as they don't have the military capability or the expertise to wage open warfare. But terrorism from without, coupled with substantial populations of Muslims inside western countries (over 2 million in US; over 6 million in France; some 350,000 in/around Sydney)are a threat that the west is still largely blind to.
But is it possible for Islamic nations to become a superpower? Maybe - all its takes is for the US to become weaker and for a revised definition of what a superpower is. Indonesia (the most populous Muslim country with some 120 mill people has a substantial standing army or close to 200,000) could in theory overrun Australia / New Zealand to start with. While at present Australia has superiority in air and sea capability, the gap is not that great. New Zealand has no air force and tiny navy.
America did take up the challenge post 9/11 and the limits of military power have been starkly exposed.
Is America still the undisputed superpower? No. American economic power is also slipping fast. All hail the WTO and unbridled pursuit of short term, short sighted profit that ignores national interests !