Growing income disparity doesn’t presage a new labor movement at home — but it may signal more terrorism for us abroad. In 1914, Henry Ford paid his factory workers $5 a day, twice the going rate, with the aim of creating a broad middle class able to buy the cars they were building. Today, that project isn’t faring so well: The Economist reports that in the U.S. “the gap between rich and poor is bigger than in any other advanced country.” And it’s growing. According to the Congressional Budget Office, from 1979 to 2001, the after-tax income of the...