The next phase of the current crisis may begin when the public begins to reassess the credibility of the monetary and fiscal measures that the Obama administration has taken in response. Neither interest rates at zero nor fiscal stimulus can achieve a sustainable recovery if people in the United States and abroad collectively decide, overnight, that such measures will lead to much higher inflation rates or outright default. As Thomas Sargent, an economist who pioneered the idea of rational expectations, demonstrated more than 20 years ago, such decisions are self-fulfilling: it is not the base supply of money that determines inflation but the velocity of its circulation, which in turn is a function of expectations. In the same way, it is not the debt-to-GDP ratio that determines government solvency but the interest rate that investors demand. Bond yields can shoot up if expectations change about future government solvency, intensifying an already bad fiscal crisis by driving up the cost of interest payments on new debt. Just ask Greece -- it happened there at the end of last year, plunging the country into fiscal and political crisis.
Finally, a shift in expectations about monetary and fiscal policy could force a reassessment of future U.S. foreign policy. There is a zero-sum game at the heart of the budgetary process: if interest payments consume a rising proportion of tax revenue, military expenditure is the item most likely to be cut because, unlike mandatory entitlements, it is discretionary. A U.S. president who says he will deploy 30,000 additional troops to Afghanistan and then, in 18 months' time, start withdrawing them again already has something of a credibility problem. And what about the United States' other strategic challenges? For the United States' enemies in Iran and Iraq, it must be consoling to know that U.S. fiscal policy today is preprogrammed to reduce the resources available for all overseas military operations in the years ahead.
Defeat in the mountains of the Hindu Kush or on the plains of Mesopotamia has long been a harbinger of imperial fall. It is no coincidence that the Soviet Union withdrew from Afghanistan in the annus mirabilis of 1989. What happened 20 years ago, like the events of the distant fifth century, is a reminder that empires do not in fact appear, rise, reign, decline, and fall according to some recurrent and predictable life cycle. It is historians who retrospectively portray the process of imperial dissolution as slow-acting, with multiple overdetermining causes. Rather, empires behave like all complex adaptive systems. They function in apparent equilibrium for some unknowable period. And then, quite abruptly, they collapse. To return to the terminology of Thomas Cole, the painter of The Course of Empire, the shift from consummation to destruction and then to desolation is not cyclical. It is sudden.