This week marks an ignoble milestone: America’s national debt is crossing $40 trillion for the first time, months earlier than was forecast. Although this exact threshold is more symbolic than intrinsically significant (there are other, more critical alarms flashing), now is as good a time as any to reflect on how we got here.

Economists prefer to work with debt held by the public when studying this issue, rather than the headline total, which includes debt the federal government effectively owes to itself. (Among those considered “public” are banks, investment funds, the Federal Reserve, and foreign countries.) By this more relevant measure, our debt stands at the slightly less mortifying figure of $32.2 trillion, or just less than the size of the whole U.S. economy. This year, the federal government will spend a projected $931 billion to service this debt, or around 3 percent of national income. Interest costs are now the third-greatest federal expenditure, behind Medicare and Social Security. It, along with the underlying debt, will only balloon from here.

Believe it or not, in 2001, the Congressional Budget Office (CBO)........

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