Global capital markets, now the most powerful force on earth, are rapidly losing confidence in the financial coherence of the 17-nation euro zone. A market implosion there, like that triggered by Lehman Brothers collapse in 2008, may not be far off. Not only would that dismantle the euro zone, but it could also usher in another global economic slump: in effect, a second leg of the Great Recession, analogous to that of 1937. This risk is evident in the structure of global interest rates. At one level, U.S. Treasury bonds are now carrying the lowest...