We were taught in Economics 101 that countries could not for long sustain large, ever-growing trade deficits. At a point, so it was claimed, the spree of the consumption-happy nation would be braked by currency-rate adjustments and by the unwillingness of creditor countries to accept an endless flow of IOUs from the big spenders. And that's the way it has indeed worked for the rest of the world, as we can see by the abrupt shutoffs of credit that many profligate nations have suffered in recent decades.