This has been an ongoing claim since the Republic was formed over 231 years ago. You'd think if this were true we'd never have attained greatness to begin with, wouldn't you?
Have there been disturbing changes in our country over the years? Without a doubt. Many of those changes have been for the good. Many have not. But America has endured, somehow.
American "decline" is the foreign-policy equivalent of homelessness: The media only take note of it when a Republican is in the White House. Broadly speaking, declinists divide between those who merely accept America's supposed diminishment as a fact of life, and those who celebrate it as long overdue. As for the causes of decline, however, they tend to agree: declining (relative) economic muscle, due in large part to the rise of China; an overextended military bogged down needlessly in Iraq and endlessly in Afghanistan; the declining value of America's "brand" on account of Bush administration policies on detention, pre-emption, terrorism, global warming -- you name it.
Yet each of these assumptions collapses on a moment's inspection. In his 2006 book "Überpower," German writer Josef Joffe makes the following back-of-the-envelope calculation: "Assume that the Chinese economy keeps growing indefinitely at a rate of seven percent, the average of the past decade (for which history knows of no example). . . . At that rate, China's GDP would double every decade, reaching parity with today's United States ($12 trillion) in thirty years. But the U.S. economy is not frozen into immobility. By then, the United States, growing at its long-term rate of 2.5 percent, would stand at $25 trillion."
If that's but one example of American decline, then I say let's see more of it!