Actually my nuke comment was in response to that guy that said the Navy protected the continental US from invasion.
Now, the US don't need to be an Empire. Much less a global one. Look at China: they're faring pretty well without owning every single seas out there. That's the sad part: the US got the costs of an Empire (Huge armies, everyone hates you) with almost none of the benefits (no colonies bringing in cash). Even when you invade a country, you don't get the spoils (look at China building mines in Afghanistan).
To be fair, the only "measured response" you get with a strategic nuclear deterrent is whether you want to blow up one city or a hundred, and in the case of an invasion, there is significant benefit in being able to repel a hostile invasion without setting off global thermonuclear war. For situations in which you don't want to blow up a city, some conventional force capability is quite helpful. The ability to park a carrier force in the Persian Gulf and threaten a blockade whenever Iran and Saudi Arabia and/or Israel start getting persnickety at each other is far more invaluable than threatening them with the destruction of Teheran, since it permits a more reasoned, rational reaction to military circumstances that serve American interests but do not necessitate a nuclear response. Whether one needs a fleet larger than the second to fourteenth-largest fleets combined to do that is a whole 'nother story, but conventional forces do have their place in the modern defense of any nation.
China's kind of a funny case. They may be doing well enough with not owning every single sea out there right now, but that doesn't mean they don't want to play the same game, if for no other reason than to protect their foreign investments in Africa and south Asia. Just one look at
their claims in the South China Sea definitely suggests something in the way of Chinese ambitions, especially given all of the hopes of oil in the region. They're also not particularly content with the way things are as far as their naval force projection is concerned; they've been attempting to build up a blue-water fleet for some time, including the recommissioning of the Varyag last year and ongoing efforts to produce a domestic aircraft carrier.