Empires rise and fall from Greek and Roman times down to the British and the French to the present-day U.S. Some lasted for centuries others mere decades. There are always new powers that want to rule the roost. For many, military power is closely linked to economic well-being. But once the cash register starts emptying overall decline is not far behind. It's clear that in the 21st-century global dominance is a contest between the U.S. and China. Russia is dangerous but is relatively weak, witness its ineptitude in Ukraine. The National Intelligence Council is a top-level U.S. government agency that produces reports projecting global trends. Not surprisingly, it identifies Beijing as a "potential rival" to Washington that "will probably have the largest economy, surpassing that of the U.S. a few years before 2030."