If at all there will be a downfall, it will be due to excessive military spending. But it looks unlikely , as the US GDP in trillions is strong enough to absorb such spending, atleast in the foreseeable future.

The downfall is society itself. We are killing one another off daily, in every city and town in ever State. We overindulge in drugs and alcohol, and with that comes crimes. Men are killing their pregnant spouses, a woman is beaten every 4 minutes by the hands of the man who "loves" her. A woman is raped every 6 seconds. Children are taken from the street, their homes, and beds, only to be molested and or killed. The economy is declining due to so many single parents trying to work and care for their children. We are worse than a 3rd world Country sometimes..

If we go to far either to the right or the left. Moderation is the key to our success. Every time we have gone too far in either direction something terrible has happened to bring us back to the middle ground.

Time. When your on top no other place to go but down.

The supervolcano under Yellowstone National Park. At some point it will blow (anytime from next week up to thousands of years off) and the worst cases scenario is an eruption for several weeks that will do a lot of damage to a lot of infrastructure and the economy and tragically claim many lives.

Picking our leaders based on "whose the best looking" or "whose the best liar" - in addition to some of the excellent answers already offered.

America will be the downfall of America... how's that for simplistic? We will implode through the degradation of our very own society... Its like a couple that had nothing... worked out of its poverty and finally thrived through sweat, blood and tears... the parents know what it took to get there.... the children on the other hand will have had everything given to them. To us (the children) that's what's normal. To poorly quote Micheal Crichton, Malcom in the film says to Hammond that he is "standing on the backs of geniuses and taking the next step without considering the consequences. You didn't earn anything for yourself so you dont know what the risks could be." In other words, we look not at what it took to get what we have now... only that we have it and want more. America started as a unified vision a common goal that pulled us together... Now its everyone for themselves and who are you to get in my way. It takes a terrorist act to unify us now... and look how long that lasts.

terrorists

Ignorance fostered by poor education and naivete. By this I mean a country-wide deficiency in worldly wisdom and informed judgment. Actually, I don't believe America is headed for any kind of imminent downfall. Rather I think it has fallen and is on its way back up after scuffing itself up pretty well. I cite the reasons stated above.

over confidence. look at 9-11. the government made it seem to the world as though america was bulet proof (so to speak) and that no one could attack them sucessfully. although america pulled through 9-11 i would still call it a sucessful attack and an acknowledgmet to america that no one is invincible. hopefully it doesnt get to that point ever again but there will always be crazies int he world and as long as America continues to be a superpower, and the more confidece they display they will unfortunately continue to be a target of world terror.

Overconsumption. The average American uses the same amount of energy as a blue whale. Ignorance. Your education system is a joke on the world stage. Bad politics. The "War against terror" has been terribly counter-productive. Division and inequality. The creation of a substantial and heavily-armed underclass may be seen as a poor idea by future generations. Exceptionalism. Believing America is somehow special and is not susceptible to the same rules that governed the Roman Empire and the British Empire. May be the biggest one of all.

generosity

I was just telling my children the other day that I felt the end of the world was at hand and sooner than most would anticipate - I feel people as a whole are the ones responsible for the destruction of our universe let alone America...People...We are our own worst enemy!

Their eagerness to jump in and sort other people's problems out, whether the result is better nor not, they still continue. Example:- That American soldier shouting at the upset Iraqi Civilians telling them that he's their to secure their freedom and basically they should be grateful. What is freedom is you have no home, money, food, security or future and your family is scattered across the area frightened and hungry. I know some of you may disagree with this, but I think you have to ask yourself this were things better before(possibly oppression but more stabilty) or now (the chaotic gap between the old order and forming of the new one)? Britain doesn't escape this one either, Yes Blair it does look like you are Bush's lapdog following him around and agreeing with whatever he says. Or is this all dreamed up by the media which self-pleasures itself in it's corporate dream.

America itself, in all it's excesses.

Apathy or Hubris, I'm not sure which and I'm too important to be bothered with deciding...

Apathy. Americans are in tune with what's going on in the world -- wars, political struggles, poverty, global warming, environmental pollution, over-development and over-population, energy consumption -- but we generally do nothing about it. You can find the typical American driving his SUV (hemi-powered) to eat a huge meal having left his lights on at home. Then he'll complain about the price of fuel and our incompetent politicians over a beer at the bar rather that getting out and voting. It's unflattering, but the fact is, not enough of us do much to affect the future of our country or the world.

Our arrogance

BUSH

4 more years for Bush! Thankfully that can't happen.

The moral degredation of society Massive amounts of unchecked illegal immigration incredible deficits that no one seems to want to curb Timidity in foriegn policy Pick one!

I honestly thought it would be our oh-so-eloquent leader, George W., but it turns out he didn't screw it up as bad as I thought he would. I guess my next guess will be our inability to not spend money that we don't have.

BUSH

Interest in and lack of education. We don't learn from the past and from the looks of it, too many people can't even spell let alone make sense when they try to communicate using the written word. We are condemned to repeat that which we do not remember(or learn from).

Nothing. America will go on bullying the rest of the world, taking what it wants (mostly that appears to be oil) and starting wars when it wants. Possibly, I think the biggest future (threat) to America is the rapid industrialisation and growth of China, which as we know is a largely communist country. we could see a new 'cold war' - not because it's merited but because the only way for western governments to effectively rule their people is by ensuring they are in a constant 'state of fear' mode. Think about it, after the (first) cold war finished western society has been desperately looking for a global threat big enough to 'panic the people'. now there's no real justification for the global threat of terrorism (and yes, I know it still exists) the american government will need a new 'threat'.

Arrogance and inertia - the failure adapt or see the need for it.

Americas leader will try to expand and will provoke a war against them and the leader will be not be able to defend it.

Greed :)

The people

I SAY ONLY TWO THINGS ARE GOING TO CRIPPLE OUR GREAT NATION: 1. Illegal immigrants- we spend hundreds of billions of dollars a year to give illegal aliens, escpecially from Mexico, social security and welfare, and now we want them to be citizens and vote???? the only way we could ever achieve such a thing is to raise taxes by 50%!! they are draining our economy greatly. people wonder why we are in such a recession right now... open your dam eyes! 2. Hate and Racism- I admit ive thought and said racist ideas, i admit ive been unpatriotic. People need to set aside their differences some way in another, and just ditch the whole race and religion issue. Im a White Christian, and i do not mind other religions one but, nor other races. I think it should be the least of humanity's concerns. Also, people need to get off their butt and start doing things, stop collecting welfare, and go do something that benefits the country. We need to find a common bond of patriotism, and we need to determine what is best for our well being- no matter the cost. As an extreme example, Germany under Hitler was so strong because of a huge drive of Nationalism and Patriotism. These things should be in every American, along with a strong work ethic. Although Hitler was a horrible person, he shows us how we need to be strong and find Patriotism deep within our souls. - a good American

Not gonna happen!