The United States is a very exceptional country, in that it was originally founded on ideas, mostly. I'm sure you've heard about the "American way" and the right to "life, liberty and the pursuit of happiness". The American people are capable of great things, and the 20th century was indeed theirs.
This century, however, seems a little more threatening, with the war on terror and a weaker economy. The rise of China also means there is another big competitor in the world stage. To make matters worse, their country also has a little of an image problem currently.
Sadly, I find that the instinct is to close up, to keep on ignoring what is happening in the rest of the world, and to refuse to understand how others see what is going on. Hardball negotiation doesn't work anymore.
Thankfully, there is also a group of people who understand that the US cannot afford to do that.