The plunge in the dollar has turned normally calm voices strident and fearful. A weak currency, they say, spells catastrophe for the U.S. economy. But like much conventional wisdom, this isn’t true. Nor is it true that the dollar, to use one favorite recent word, has “collapsed.” You wouldn’t know it, however, from recent headlines. This week’s Economist magazine, known for its cool-headed discussion of economic events, has this on its cover: “The Panic About the Dollar.” Others see in the dollar’s slump a metaphor for America’s future — one of decline and waning influence in the world. To be...