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History strongly supports the proposition that major financial crises are followed by major fiscal crises. “On average,” write Carmen Reinhart and Kenneth Rogoff in their new book, This Time Is Different, “government debt rises by 86 percent during the three years following a banking crisis.” In the wake of these debt explosions, one of two things can happen: either a default, usually when the debt is in a foreign currency, or a bout of high inflation that catches the creditors out. The history of all the great European empires is replete with such episodes. Indeed, serial default and high inflation have tended to be the surest symptoms of imperial decline

Why should we fear rising real interest rates ahead of inflation? The answer is that for a heavily indebted government and an even more heavily indebted public, they mean an increasingly heavy debt-service burden. The relatively short duration (maturity) of most of these debts means that a large share has to be rolled over each year. That means any rise in rates would feed through the system scarily fast.

Already, the federal government’s interest payments are forecast by the CBO to rise from 8 percent of revenues in 2009 to 17 percent by 2019, even if rates stay low and growth resumes. If rates rise even slightly and the economy flatlines, we’ll get to 20 percent much sooner. And history suggests that once you are spending as much as a fifth of your revenues on debt service, you have a problem. It’s all too easy to find yourself in a vicious circle of diminishing credibility. The investors don’t believe you can afford your debts, so they charge higher interest, which makes your position even worse.

This matters more for a superpower than for a small Atlantic island for one very simple reason. As interest payments eat into the budget, something has to give—and that something is nearly always defense expenditure. According to the CBO, a significant decline in the relative share of national security in the federal budget is already baked into the cake. On the Pentagon’s present plan, defense spending is set to fall from above 4 percent now to 3.2 percent of GDP in 2015 and to 2.6 percent of GDP by 2028.

The precedents are certainly there. Habsburg Spain defaulted on all or part of its debt 14 times between 1557 and 1696 and also succumbed to inflation due to a surfeit of New World silver. Prerevolutionary France was spending 62 percent of royal revenue on debt service by 1788. The Ottoman Empire went the same way: interest payments and amortization rose from 15 percent of the budget in 1860 to 50 percent in 1875. And don’t forget the last great English-speaking empire. By the interwar years, interest payments were consuming 44 percent of the British budget, making it intensely difficult to rearm in the face of a new German threat.