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This month marks the second anniversary of September 15, 2008, a date that will be remembered as one of the worst moments in the history of the global economy. On that day US investment bank Lehman Brothers collapsed, triggering a major financial crisis and dramatically worsening a worldwide recession whose effects are still being felt.

The date also matters in the history of international politics, accelerating what is destined to be the most important international political trend of the second decade of the 21st century: the growing financial obligations of the United States government. Coping with these obligations will limit the resources available for American foreign policy, thereby reducing the nation's international role. Because that role is so important - the United...