I'd say our decline is primarily the result of over-spending on the War on Terror, and increasing our national debt by significant margins in the past 10 years - we've added about 10 trillion to the debt in a little over 10 years, which is horrible.
Most of our problems, though, could be rectified very quickly with the right people in power. I don't think our decline is a serious one - just one that needs addressed with a few measures.
China's rise doesn't mean the US is declining. It just means they are rising. However, they have a lot more deep-rooted problems than we do. They are not making friends in the area, as they are harassing the Indians, Vietnamese and Fillipinos due to the Spratly issue.
In addition, the reality is that we're evolving into a tri-polar future in regards to superpowers and alliances. It will be similar to the one we saw immediately after the Sino-Soviet collapse of the 60s, but instead of Moscow, it will be New Delhi.
Additionally, as mentioned somewhat, the backlash against Chinese aggression and force progression will only get more vocal. ASEAN will be a very prominent force in the future, especially backed by the rapidly advancing Korean economic complex. The Indonesians have recently signed onto a joint venture with South Korea on a 4.5 or 5th generation fighter, which is pretty much unheard of outside of Beijing and Russia. Likely, such armaments will spread throughout the region, fortifying the enemies of China, which will counter their influence.
Why say all of this? Because it shows that China has a lot of struggles ahead. They are a superpower with superpower problems, but few superpower solutions that America has been afforded - namely democracy and a general history of non-aggression. Both will hurt China immensely in the next 20 years.
In the end, we're not declining. Other nations are increasing, and there is nothing wrong with that. The question is what will we and others do with the power that we wield.
Back from the dead, I'm afraid.
