This chart posted at Business Insider last week shows how the British, American, Russian and Chinese empires have risen and declined over the last 200 years. It indicates that China is on a steady ascent and will overtake the US in the next few years.
However, if you were to extend this chart back another half century, and to include lines for France and Germany, you would see that France had surpassed Britain before its failed attempt at hegemony that ended with Napoleon’s defeat. And you would then see that Germany had surpassed both Britain and the US before it’s own hegemonic bid had been finally defeated after two world wars. So the relative decline of the lead economy does not automatically presage global ascendancy by its replacement. Indeed, when we examine the hegemonic cycles of the modern era, we see that the victor in hegemonic struggles always has a more open society than the failed challenger, and it is always on the periphery of the World Island, not at the core.
Of course, this does not guarantee that the US will continue its hegemony; an outside contender such as India might eventually take up leadership of the world system (nor, of course, does it prove that China is doomed to fail). What it does indicate is that the final phase of our current cycle is near.