When Interest Payments Overtake Defense, History Has a Warning

The United States now spends more servicing its national debt than it spends on national defense, a threshold it last crossed nearly a century ago and has only recently crossed again. Net interest payments are on pace to reach roughly a trillion dollars this fiscal year, ahead of the defense budget, and the gap is projected to widen for years to come. The headline framing treats this as a budget arithmetic problem. The historian's framing, associated with a rule sometimes called Ferguson's Law after the scholar who documented it across centuries of great powers, treats it as something closer to a warning light, one that has flashed before major shifts in a nation's geopolitical standing.

The Historical Echo

The pattern has appeared repeatedly across history, and one of the clearest examples involves a power that, on paper, should have been immune to it. Habsburg Spain in the sixteenth and seventeenth centuries controlled the silver mines of Potosi and the gold flowing back from its American territories, an inflow of physical precious metal that no other European power could match. Yet that same Spain declared sovereign bankruptcy repeatedly, under Philip II alone in 1557, 1560, 1575, and 1596, with further defaults following in 1607, 1627, and 1647. By the early seventeenth century, debt service was consuming something like 40 to 50 percent of the crown's annual revenue.

The paradox resolves once the mechanism is understood. Spain did not simply spend its silver as it arrived. It borrowed heavily against future treasure fleets, often through Genoese banking houses, to fund wars against France and the Ottomans years before the actual bullion reached Seville. The physical gold and silver were real, but the crown's financial commitments outran even that extraordinary flow of hard assets, and when a fleet was delayed by weather or war, the entire structure of borrowed promises came due at once. Spain's downfall was not a shortage of physical wealth. It was a mismatch between the pace of real, tangible resources arriving and the pace of paper obligations layered on top of them, a distinction that mattered more than how much gold sat in the holds of the ships.

Where Patient Capital Is Positioning

The lesson from Habsburg Spain is not that physical assets are worthless once debt gets large enough. It is that even a nation rich in tangible resources can undermine itself by borrowing against the future faster than those resources can be delivered, and that the mismatch tends to surface exactly when debt service starts crowding out the spending that once demonstrated national strength. The current moment in Washington, where interest payments now exceed the defense budget for the first time in generations, sits squarely in the category of warning signs that Ferguson's research associates with previous transitions in great power status, from Habsburg Spain to the late Ottoman Empire to the British Empire before the World Wars.

For a family or institution thinking across decades, the takeaway is not a prediction about exactly when or how this resolves. It is a reminder that a government's fiscal position, not just its currency or its bond market, eventually shapes what that government can and cannot do, and that physical assets held directly, gold, silver, land, and productive energy infrastructure, do not carry the same dependency on a sovereign's ability to keep servicing an ever-growing pile of paper promises. Spain's mines never ran dry. Its finances did. That distinction is worth sitting with.

It is also worth noting what did retain its value across every one of those Spanish bankruptcies. The silver and gold that reached Seville, once in private hands, kept its purchasing power regardless of how many times the crown itself defaulted on its creditors. Sovereign bankruptcy wiped out bondholders repeatedly across those decades. It did not, and structurally could not, wipe out the metal itself. That is the same distinction quietly at work in a government's finances today, and it is why long-horizon capital keeps returning to physical assets whenever the gap between debt service and the spending that once defined national strength starts to widen.

The Capital Memo