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The U.S. national debt grew because the federal government repeatedly spent more than it collected in revenue and borrowed to cover the difference. Wars, recessions and emergency responses drove major increases, while persistent gaps between spending and revenue—and now rising interest costs—have kept adding to the total. No single war, law, party or president accounts for the whole debt.
What the national debt is—and what it is not
A deficit is the amount by which federal outlays exceed receipts during a fiscal year. Debt is the accumulated borrowing balance. When the government runs a deficit, it generally borrows to finance it; repeated deficits add to debt. A surplus can reduce debt held by the public. The U.S. fiscal year ends September 30, so figures labeled FY 2025 cover the year ending on that date, not calendar 2025. Treasury’s national debt overview explains the distinction.
Debt held by the public versus gross federal debt
Debt held by the public is Treasury borrowing held outside federal government accounts, including by investors and other holders. Gross federal debt includes that amount plus Treasury securities held in government accounts, such as trust funds. Those intragovernmental securities are not the same as debt held by outside investors. The Congressional Budget Office (CBO) often focuses on debt held by the public when analyzing borrowing’s effects on interest rates and private investment. Treasury’s FY 2025 financial report separately stated that $30.3 trillion in debt and interest was payable at September 30, 2025; that balance-sheet figure should not be treated as interchangeable with debt held by the public or debt subject to the statutory limit. CBO’s 2026 budget outlook and Treasury’s FY 2025 financial report explain their respective measures.
Debt is not the debt limit
The statutory debt limit constrains Treasury’s ability to borrow to meet obligations already authorized under law. Raising or suspending it does not itself authorize new spending or create the obligations that produced the debt. Treasury makes that distinction in its FY 2025 financial report.
Debt-to-GDP is a different measure
A dollar figure measures nominal borrowing; debt as a share of gross domestic product (GDP) compares borrowing with the size of the economy. The ratio can rise because debt grows, GDP falls or grows more slowly, or a combination of those factors. Comparisons over time should specify both the debt measure and whether the figure is nominal or a share of GDP.
How successive events and budget choices built the debt
The debt accumulated across many eras. Treasury’s historical overview identifies borrowing for the Revolutionary War, the Civil War and World War I, as well as the Afghanistan and Iraq wars. It also points to the Great Recession and the COVID-19 pandemic as more recent periods of sharp increases. Such episodes can require large outlays quickly, while crises may also reduce tax revenue.
Wars, recessions and emergencies
Wars and national emergencies can push spending sharply higher. Recessions tend to squeeze the budget from both sides: tax receipts may fall as incomes and employment decline, while demand for government support can rise. Treasury identifies stimulus, increased spending and revenue losses during widespread unemployment among broad contributors to sharp debt increases. These are explanations of the mechanisms, not a quantified breakdown of today’s debt by event.
Persistent gaps between spending and revenue
Debt does not rise only during dramatic crises. If federal outlays exceed receipts year after year, the resulting borrowing accumulates even when no single event dominates the budget. Tax policy, spending decisions and economic conditions all affect the gap. The historical sources identify broad drivers but do not allocate the current total among individual laws, administrations or events.
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Interest is a cost of financing accumulated debt, and it can itself contribute to future borrowing. CBO says interest costs depend mainly on the amount of debt held by the public and the average interest rate on that debt. When a deficit includes interest payments, borrowing to cover those payments increases the debt on which later interest may be due. The effect is not automatic at a fixed rate: both the debt balance and interest rates matter.
What the latest figures show
The figures below distinguish observed FY 2025 results from CBO’s conditional baseline projections published in February 2026. The projections are not final outcomes or certainties; they reflect specified laws and economic assumptions. CBO’s FY 2025 budget review reports the observed figures, while its 2026 budget outlook gives the baseline projections.
| Measure | Observed or projected figure | What it means |
|---|---|---|
| FY 2025 federal deficit | $1.8 trillion; 5.9% of GDP | CBO reported the deficit was $41 billion, or 2%, below FY 2024. The 50-year average deficit was 3.8% of GDP. |
| Net interest in FY 2025 | More than $1 trillion | CBO reported this was the first fiscal year in which net interest exceeded $1 trillion. |
| Debt held by the public at FY 2025 year-end | 99.8% of GDP | Up from 97.4% at FY 2024 year-end. |
| FY 2026 deficit | $1.9 trillion projected | CBO’s February 2026 baseline projection, not an observed result. |
| Debt held by the public in FY 2026 | 101% of GDP projected | CBO’s February 2026 baseline projection, not a guaranteed outcome. |
| Debt held by the public in 2036 | 120% of GDP projected | CBO’s February 2026 baseline projection, conditional on its assumptions and laws in place at the time. |
Why the debt is expected to keep growing
CBO’s February 2026 baseline projects large deficits by historical standards. It identifies Social Security, Medicare and interest costs among the factors contributing to outlay growth. Those pressures help explain the outlook, but they do not by themselves explain all the debt accumulated over the country’s history.
Population aging and health-care costs
CBO connects projected growth in Medicare spending with an aging population and rising health-care costs, and identifies Social Security as another source of mandatory spending growth. These are long-term budget pressures: they affect the path of spending and deficits under the baseline, rather than serving as a single cause of the existing debt.
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Interest rates and economic growth affect the path
Higher average interest rates can make financing a given amount of publicly held debt more expensive. The debt-to-GDP ratio also depends on economic growth: a larger economy can change the ratio even if the nominal debt rises. CBO’s projections therefore depend on economic assumptions as well as laws in place when the baseline was prepared.
How to read claims about who caused the debt
It is misleading to attribute the whole national debt to one war, tax cut, party or president without a sourced quantitative breakdown. The total reflects borrowing across many years, while the annual deficit reflects the interaction of spending, revenue and economic conditions in a particular fiscal year. To assess a specific policy, look at its estimated effect on deficits and debt over a stated period and under clearly stated assumptions; a broad historical account alone cannot assign it a precise share of today’s balance.
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