Niall Ferguson: Is the sun setting on the American empire?

The Bourbon monarchy in France passed from triumph to terror with astonishing rapidity. The sun set on the British Empire almost as suddenly. The Suez crisis in 1956 proved that Britain could not act in defiance of the US in the Middle East, setting the seal on the end of empire.

What are the implications for the US today? The most obvious point is that imperial falls are associated with fiscal crises: sharp imbalances between revenues and expenditures, and the mounting cost of servicing a mountain of public debt.

Think of Spain in the 17th century: already by 1543 nearly two-thirds of ordinary revenue was going on interest on the juros, the loans by which the Habsburg monarchy financed itself.

Or think of France in the 18th century: between 1751 and 1788, the eve of Revolution, interest and amortisation payments rose from just over a quarter of tax revenue to 62 per cent.

Finally, consider Britain in the 20th century. Its real problems came after 1945, when a substantial proportion of its now immense debt burden was in foreign hands. Of the pound stg. 21 billion national debt at the end of the war, about pound stg. 3.4bn was owed to foreign creditors, equivalent to about a third of gross domestic product.

Alarm bells should therefore be ringing very loudly indeed in Washington, as the US contemplates a deficit for 2010 of more than $US1.47 trillion ($1.64 trillion), about 10 per cent of GDP, for the second year running. Since 2001, in the space of just 10 years, the federal debt in public hands has doubled as a share of GDP from 32 per cent to a projected 66 per cent next year. According to the Congressional Budget Office's latest projections, the debt could rise above 90 per cent of GDP by 2020 and reach 146 per cent by 2030 and 344 per cent by 2050.

These sums may sound fantastic. But what is even more terrifying is to consider what ongoing deficit finance could mean for the burden of interest payments as a share of federal revenues.

The CBO projects net interest payments rising from 9 per cent of revenue to 20 per cent in 2020, 36 per cent in 2030, 58 per cent in 2040 and 85 per cent in 2050. As Larry Kotlikoff recently pointed out in the Financial Times, by any meaningful measure, the fiscal position of the US is at present worse than that of Greece.

For now, the world still expects the US to muddle through, eventually confronting its problems when, as Churchill famously said, all the alternatives have been exhausted. With the sovereign debt crisis in Europe combining with growing fears of a deflationary double-dip recession, bond yields are at historic lows.

There is a zero-sum game at the heart of the budgetary process: even if rates stay low, recurrent deficits and debt accumulation mean that interest payments consume a rising proportion of tax revenue. And military expenditure is the item most likely to be squeezed to compensate because, unlike mandatory entitlements (social security, Medicaid and Medicare), defence spending is discretionary.

It is, in other words, a pre-programmed reality of US fiscal policy today that the resources available to the Department of Defense will be reduced in the years to come. Indeed, by my reckoning, it is quite likely that the US could be spending more on interest payments than on defence within the next decade.

And remember: half the federal debt in public hands is in the hands of foreign creditors. Of that, a fifth (22 per cent) is held by the monetary authorities of the People's Republic of China, down from 27 per cent in July last year. It may not have escaped your notice that China now has the second-largest economy in the world and is almost certain to be the US's principal strategic rival in the 21st century, particularly in the Asia-Pacific. Quietly, discreetly, the Chinese are reducing their exposure to US Treasuries. Perhaps they have noticed what the rest of the world's investors pretend not to see: that the US is on a completely unsustainable fiscal course, with no apparent political means of self-correcting. That has profound implications not only for the US but also for all countries that have come to rely on it, directly or indirectly, for their security.

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A favourite phrase of this great country (Australia) is "No dramas". But dramas lie ahead as the nasty fiscal arithmetic of imperial decline drives yet another great power over the edge of chaos.