This article about Chinese cyber attacks points out some valuable points to consider for anyone touting America's military might: http://www.nationaljournal.com/njmagazine/cs_20080531_6948.php
First of all, there's no question America is "mighty" by any measure of conventional military capability.
The Army and Marine Corps are strained by Iraq right now, and dangerously degraded, but even degraded they are far-and-away more capable than almost any other ground military force on the globe. The only armies with comparable capabilities are firm allies. The main limitation on American ground capability is size. Many ground scenarios require a lot of boost on the ground and there the U.S. capability is always limited (and more so because of the war)
In the air and at sea, however, there is simply no contest. There are no air forces in the world that are not allies that have a prayer of successfully defending their air space. There are no navies afloat, including allies, that can contest anything more than their immediate coastal waters. When Britain "ruled the waves" it was content to have a Navy that equalled the next two biggest. The U.S. Navy is larger (not counting minor ships) than all the rest of the world's navies put together. No one since Imperial Rome has had that level of superiority.
Aside from the depredations of submarines, there's no nation that can fight us at sea. While subs are worrisome, history suggests that they cannot be decisive alone. The only successful submarine campaign (U.S. vs. Japan) was won by the stronger navy against the weaker one. Attempts by the weaker naval power to parley a sub campaign into strategic success fell short (Germany in two world wars, Japan in WW2).
The fate of Iraq in 1991 and 2003 is not lost on our potentiall opponents either.
No nation can challenge us in conventional warfare with any hope of success. Being, however, unwilling to simply surrender their interests and let the U.S. humiliate them, bully them and otherwise have its way, countries with interests opposed to ours will obviously search for other ways to fight.
It would be a mistake to underestimate the creativity of potential opponents, especially those with long military traditions such as Iran and China. While their economic power doesn't allow them to match the U.S. in conventional military capability, both of those nations have very long military traditions independent of the Western one.
The Iranians, for example, no doubt noted that an American admiral was able to use unconventional tactics to neutralize a Navy battle group in a notorious wargame. The admiral resigned when the game was rigged and restarted to void the defeat. Have the recent incidents involving small boats stalking U.S. warships in the Persian Gulf been tests of that tactic? Possibly. It would be wise to consider the probability that an attack on Iran might unleash incidents of unconventional was and terrorism in widely separated areas of the world. Still, while the Iranians can hope to achieve some embarrassing successes, they represent an inherently limited threat. Even the loss of an aircraft carrier, while an enormous embarrassment and a naval tragedy, would not change the relative balance of power one bit. Details: http://www.iht.com/articles/2008/01/12/america/12navy.php?page=1
Much more dangerous is China, which is a continental-sized power that is growing in economic might. The Chinese are still a couple of generations, at least, from being able to match the U.S. in conventional military arms -- if they were even interested in doing so. But the Chinese are heirs to a military tradition that stretches back several thousand years. One that never had a "dark ages" that broke its connection to classical times. That Chinese military tradition has always emphasized the value of outthinking, out maneuvering and surprising the foe. Western military thought has tended to regard many tactics as too "sneaky" or "ungentlemanly" for real warriors. Unconventional warfare has posed many challenges for westerners because it rarely results in stand-up "fair" fights. Chinese military thought doesn't really make a distinction between "unconventional" and "conventional" fighting. To the Chinese all warfare involves yin and yang, the interaction between the ordinary and the extraordinary fighting elements. And importantly, yin and yang, ordinary and extraordinary, are not inherent properties of the force. Depending on circumstances the yin can become the yang. The extraordinary can become the ordinary. A general might mass his armored forces on the plain to fix the attention of the enemy while some light infantry flanks through difficult terrain to cut off a retreat.
This directly relates to the cyber skirmishing that's been going on for some years now between hackers suspected to be Chinese and various American business and government computer systems.
The Chinese, knowing that they can't compete with bombs, aircraft carriers and missiles, may be seeking to turn the conflict into one they can compete in -- a war of cyber attacks, anti-sat actions, mines, subs and terror.
They're teasing around with conventional surface naval capabilities, but very seriously building a significant sub force. There's no reason to think they would try to emulate the U-boat campaign with those subs. There's no reason to think that the U.S. would be vulnerable to a sea interdiction campaign in any sort of useful time frame for a Sino-American conflict. But subs could be used as platforms for other kinds of attacks and raids that could be very damaging.
The Chinese have demonstrated an interest in anti-sat weapons. The U.S. is heavily reliant on satellites for all sorts of uses and it could take many years to rebuild our networks if they were destroyed.
The overseas Chinese community is enormous and the Chinese have had some success in recruiting among those populations to further their interests. In a conflict one should expect some support from them, which could take many, many forms -- not necessarily direct action.
And finally, the cyber threat. Apparently there are many ways that attacks through the Internet could be waged. The very characteristics that make the Internet so useful also make it very vulnerable. The Y2K threat turned out to be minimal, but looking back at those concerns points out the ways that the system could be vulnerable -- maybe not to bad code, but to hostile action.
If a conflict were to break out between the U.S. and China, it will take some unexpected directions.