By the time President Harry Truman left office in 1953, the United States had spent the better part of a decade paying down the debt it had run up to win World War II. The peak—reached in 1946—was 106% of GDP, a number so large that policymakers spent a generation treating it as the high-water mark of what the country could survive.

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That mark is about to fall.

According to a new chart book, Spending, Taxes, and Deficits: A Book of Charts, released in April by Jessica Riedl, a budget and tax fellow at the Brookings Institution, the federal debt held by the public is projected to reach 137% of GDP within a decade—blowing way past the World War II peak. It’s what what budget analysts now call a “current-policy” baseline that assumes the extension of expiring tax cuts and roughly stable discretionary spending. The gross national debt itself crossed $39 trillion in March in less than five months after it hit $38 trillion, and is what the Peter G. Peterson Foundation called a “staggering” pace of accumulation with few precedents outside wartime.

“Borrowing trillion after trillion at this rapid pace with no plan in place is the definition of unsustainable,” Peterson Foundation CEO Michael Peterson told Fortune when the milestone hit.

The deficit’s center of gravity has shifted

For most of the postwar era, deficits were episodic. They blew out during recessions and wars and contracted during expansions. But now, the 2026 deficit is on track to hit 5.8% of GDP—at a moment of relative peace and full employment—and Riedl’s current-policy baseline projects it climbing to roughly 9% of GDP by 2036, a level the U.S. has otherwise reached only during crisis: the Great Depression, World War II, the 2008 financial crisis, and the COVID pandemic.

In nominal terms, the annual deficit is projected to grow from roughly $2 trillion today to $4.4 trillion by 2036.

What’s driving it isn’t a mystery. Medicare alone is on track for a $109 trillion cash shortfall over 30 years—79% of the entire projected deficit. The typical retiring couple, Riedl notes, will receive about $3 in Medicare benefits for every $1 they paid into the system.

Riedl’s data in the 132-page book show that nearly the entire $2.3 trillion increase in annual deficits between 2023 and 2036 is attributable to rising shortfalls in just two programs: Social Security and Medicare. This is a reason, not an excuse: the Congressional Budget Office projects $138 trillion in cumulative deficits through 2056, but if you strip out Social Security and Medicare, the rest of the federal budget actually runs a $19 trillion surplus over that period. The two retirement programs run a combined $157 trillion deficit.

Riedl projects Social Security and Medicare’s combined cash shortfall, including the interest costs on the debt issued to backfill them, will hit 18.4% of GDP by 2056—roughly equivalent to the entire federal budget in a normal year.

“Virtually impossible” to balance through cuts

One of the charts in the Brookings report is titled “Balancing the Budget in 10 Years on Lower-Priority Spending Cuts Alone is Virtually Impossible,” and the view is pretty gloomy. To balance the 2036 budget through equal program cuts across the board, lawmakers would need to slash every program by 36%. Keep those cuts away from Social Security and Medicare, and the required cut on everything else jumps to 69%. Take veterans’ benefits off the table too, and the figure climbs to 80%. Add defense to the protected list, and the necessary cut to whatever’s left reaches 117%. As in: cut everything else to zero, and the budget still doesn’t balance.

This is roughly the position Washington finds itself in. Discretionary spending, what Congress appropriates each year for things like defense, education, justice, transportation, and scientific research, has fallen from 12% of GDP in the early 1960s to roughly 6% today, and is projected to keep falling. Defense outlays, at 2.8% of GDP, are at their lowest share of the economy since before World War II. Cutting it to NATO’s 2%-of-GDP target would save about $4 trillion over a decade, a real number that nonetheless covers only about a fifth of the projected deficit over the same period.

Riedl told Fortune in March that recent White House proposals amount to “an historic defense spending increase coupled with fake offsets on spending and revenues” and that the resulting trajectory—annual deficits exceeding $4 trillion by 2036—is “totally unsustainable.” She added, “They can’t be the party of endless tax cuts, historic defense spending hikes, and still not touching Social Security and Medicare. Something’s got to give.”

Taxing the rich won’t close the gap, either

If cuts can’t do it, what about taxes? Riedl’s menu of tax-increase options shows that even the most aggressive proposals raise modest sums relative to the long-term shortfall.

A 10-percentage-point across-the-board hike in income tax rates would raise about 3.5% of

The $39 trillion debt is set to surpass its postwar peak—and the math says Washington can’t simply cut its way out