Debt has ALWAYS been the ruin of great powers. Is the U.S. next?

THAT is the question posed by preeminent historian Niall Ferguson in a recent thought-provoking Wall Street Journal column:

[…]  What I call Ferguson’s Law states that any great power that spends more on debt service than on defense risks ceasing to be a great power. The insight is not mine but originates with the Scottish political theorist Adam Ferguson, whose “Essay on the History of Civil Society” (1767) brilliantly identified the perils of excessive public debt.

Ferguson understood what modern economists call the “tax-smoothing” properties of public debt: By borrowing to pay for a war or some other emergency, a government can spread the cost over multiple generations of taxpayers. But he also saw the catch. “The growing burden,” he observed, is “gradually laid,” and though a nation may “sink in some future age, every minister hopes it may still keep afloat in his own.” For this reason, public debt is “extremely dangerous…in the hands of a precipitant and ambitious administration.”

His conclusion was prophetic: “An expense, whether sustained at home or abroad, whether a waste of the present, or an anticipation of future, revenue, if it bring no proper return, is to be reckoned among the causes of national ruin.”

Economists have long sought in vain a threshold that defines how much debt is too much. My own formulation of Adam Ferguson’s idea focuses our attention on the crucial historical relationship between debt service (interest plus the repayment of principal) and national security (expenditure on defense, including investment in research and development).

The crucial threshold is the point where debt service exceeds defense spending, after which the centripetal forces of the aggregate debt burden tend to pull apart the geopolitical grip of a great power, leaving it vulnerable to military challenge.

The striking thing is that, for the first time in nearly a century, the U.S. began violating Ferguson’s Law last year. Annual defense spending—to be precise, national defense consumption expenditures and gross investment—was $1.107 trillion in 2024, according to the Bureau of Economic Analysis (BEA), while federal expenditure on interest payments (the government long ago gave up on paying down principal) topped out at $1.124 trillion.

These outlays can also be expressed as percentages of gross domestic product. The Congressional Budget Office (CBO), which uses a narrower definition of defense spending than the BEA, places it at 2.9% of GDP for last year. Net interest payments (adjusting for the interest received by bonds held by government agencies) amounted to 3.1%. 

We have seen nothing like this since the era of isolationism. Between 1962 and 1989, U.S. defense spending averaged 6.4% of GDP; debt service was less than a third of that at 1.8%. Even after the end of the Cold War, the federal government was still spending, on average, roughly twice as much on national security as on interest on the debt.

The fact that the U.S. is currently projected to spend a rising share of its GDP on interest payments and a falling share on defense means that American power is much more fiscally constrained than most people realize. By 2049, according to the CBO’s latest long-term budget projection, net interest payments on the federal debt will have risen to 4.9% of GDP. If defense spending maintains its recent share of discretionary spending, it will amount to half that share of GDP. 

Nor is there any real possibility that defense spending will increase dramatically. Because such spending is discretionary, it has to be appropriated by Congress every year, unlike spending on entitlement programs (which is mandatory) and interest payments (nonpayment of which would be default). If anything, budgetary constraints are likely to put downward pressure on defense spending in the decades ahead.[…]

Ferguson hits us with a few examples from history to illustrate his point.  He points to 17th century Spain — sitting atop a global empire pieced together through many a successful military campaign.  The Spanish monarchy began to rely more and more on debt to finance its burgeoning empire.  Between 1607 and 1682, Spain defaulted on its debt five times. 

[…]  French intervention in support of the American colonists, culminating at Yorktown in 1781, may have appeared a strategic masterstroke. But the fiscal consequences took Louis XVI’s government far beyond the limits of Ferguson’s Law. In 1780, debt service absorbed two-fifths of total expenditure, the war department just a quarter. By 1788, debt service rose above half of total expenditure.

The history of the 19th century furnishes further examples: the Ottoman Empire, Austria-Hungary, Tsarist Russia. But the best example of all—and the one from which Americans have the most to learn—is that of Great Britain.

[…] The British case illustrates that defying Ferguson’s Law need not doom a great power to swift decline. Britain crossed the limit in three periods after the mid 19th century, but in each case it was able to cross back. Decline inexorably came, as inflation and low productivity growth forced successive governments to give up colonies and shrink the armed services.   […]

[…]  What are the implications for America today? Geopolitically, the U.S. finds itself in a situation comparable with that of Britain in the 1930s. Its military commitments are global, as has been true since 1945, and it confronts a new axis of authoritarian powers. 

Yet America’s fiscal position is far more constrained today than ever before. The U.S. government is now in violation of Ferguson’s Law and is likely to move further beyond its crucial limit in the coming decades.

Can the U.S., like Victorian and interwar Britain, find a way back? Can it do even better, successfully deterring its foes—as Britain failed to deter Germany—and averting the possibility of a ruinous World War III? Or is America doomed to follow Habsburg Spain, the Ottoman Empire, Bourbon France and Austria-Hungary down the path of default, depreciation and imperial decline—even revolution?

There are four important differences between Britain in the 1930s and the U.S. in the 2020s, and all of them work to America’s disadvantage.

First, the term structure of U.S. debt is shorter, making it more sensitive to changes in interest rates. That makes it inherently harder to “inflate debt away” like the U.K. after World War II. Second, much more of it is in the hands of foreign investors. Third, the trend of real interest rates in the U.S. seems less likely to be downward than it was in 1930s Britain.

Whereas British real interest rates fell in the Depression, in America they are currently projected by the CBO to rise from 1.7% in 2024 to 1.9% in 2026, declining slightly to 1.8% in 2034. The real growth rate of the economy is projected to be almost identical. In this scenario, America’s debt will cost more to service in the period 2025-2035 than it did in 2015-2025, when the average real rate was 0.3%, especially because the stock of debt will continue to grow.

Finally, the U.S. today is encumbered with an expensive welfare system designed for a society with a higher fertility rate and lower life expectancy. Entitlement programs such as Social Security and Medicare are now the biggest items of federal expenditure. They will only become more expensive as the population ages.

History suggests that any sustained period when a great power spends more on interest payments than on military capabilities is likely to see its strategic rivals challenge its position. The tension between “guns and coupons” (as the interest-bearing parts of bonds used to be known) may also undermine its domestic stability, as governments try and fail to meet the competing demands of generals, bondholders, taxpayers and welfare recipients. 

In the absence of radical reform of America’s principal entitlement programs—which successive administrations this century have either failed to achieve or ruled out—the only plausible way that the U.S. can come back within the limit of Ferguson’s Law is therefore through a productivity miracle. […]