Large-scale U.S. military interventions raise long-term Treasury yields by increasing expected federal deficits

What's this about?

People disagree about whether big U.S. wars make long-term Treasury rates rise.

The idea may be true in some cases, but it does not fit every war.

What supporters say

• Big wars can leave the U.S. with large bills for years, such as care for past troops.
• If the U.S. pays those bills by borrowing, its debt grows and long-term rates may rise.

What critics say

• We cannot tell how much of a rate change came from a war, since many forces affect Treasury rates.
• During a crisis, people may buy U.S. bonds because they seem safe, which can push rates down.

How to read this

The number of points on each side does not show which side is right; strong proof matters more.

The bottom line

The evidence shows a real path from a big war to more debt and higher long-term rates.

Still, the link is not certain, because safe-haven buying and other forces can lower rates.

The claim is that large-scale U.S. military interventions can push up long-term Treasury yields by increasing expected federal deficits. The evidence supports that as a possible chain of events, but not as a rule that applies to every intervention.

The case for

Large military interventions can create substantial and lasting federal obligations. The wars in Iraq and Afghanistan, for example, involved direct spending, additional borrowing costs and long-term veterans’ benefits. Government and independent estimates show that these costs can continue well after combat ends, making the resulting fiscal shock more than a short-term budget event. 1 (see Figure 1)

The next link is the relationship between persistent deficits and interest rates. Research on fiscal policy, along with analysis from the Congressional Budget Office, finds that rising federal debt can put upward pressure on long-term borrowing costs. More Treasury borrowing increases the supply of government debt, while large and continuing deficits may also raise concerns about inflation, crowd out private investment or increase the extra return investors demand for holding long-term bonds. 2 CBO projections say that persistent primary deficits and rising debt can increase interest costs and push interest rates higher over time, although the agency does not present a single, predictable effect for every episode (see Figure 2).

This gives the claim a credible two-stage mechanism: an intervention may increase expected federal liabilities, and those liabilities may later contribute to higher long-term yields. The effect is more likely when investors expect borrowing needs and debt to remain high, particularly when the economy is operating near capacity.

The case against

The main problem is that military intervention is only one influence on Treasury yields. During periods of geopolitical tension, investors may seek the safety and liquidity of U.S. government bonds. That safe-haven demand can push yields down, potentially offsetting or even outweighing the upward pressure from intervention-related borrowing. Global savings, international financial conditions and uncertainty about inflation also affect long-term U.S. rates. 3 (see Figure 3)

Federal Reserve policy can pull in the same direction. If the central bank keeps policy accommodative, or if investors expect weak economic growth, yields may remain low even as government debt rises. Fiscal research also suggests that the effect of expected deficits depends on the monetary system, confidence in fiscal management and whether markets believe the spending will be temporary or eventually reversed.

There is also a serious measurement problem. A rise or fall in Treasury yields around an intervention cannot, by itself, show that expected deficits caused the change. Inflation expectations, Federal Reserve decisions, recessions, global capital flows and changes in the term premium may all move at the same time. Historical Treasury data can show timing and correlation, but cannot establish what yields would have been without the intervention. 4

The available evidence does not provide a study that isolates the specific effect of a large-scale intervention on long-term yields through expected deficits. It also does not settle how announcement effects, wartime safe-haven flows, financing choices and later debt expectations interact in different conflicts.

The bottom line

The evidence favours a plausible conditional link, but only weakly supports the broader claim. Large interventions can clearly raise federal liabilities, and persistent deficits can raise long-term borrowing costs. But those links do not prove that interventions generally produce higher Treasury yields.

The most important unanswered question is whether deficit expectations outweigh safe-haven demand and other economic forces in each case. The fiscal-cost and debt-to-rate relationships are reasonably well supported, but the intervention-specific causal effect has not been directly estimated. As a result, the claim is best understood as a possible outcome that depends on the fiscal and monetary environment—not a general rule.