In 2004, Osama bin Laden explained how his terrorists were going to win its struggle against a vastly more powerful adversary: al-Qaida sought to "blee[d] America to the point of bankruptcy." Bin Laden is dead and his organization is a shadow of what it once was. Yet a new paper from a Harvard lecturer suggests that he had a point.

Linda J. Bilmes of the Harvard Kennedy School estimates that the wars bin Laden provoked the U.S. into launching over the past decade have cost "somewhere between $4 and $6 trillion." She reaches that staggeringly high total by calculating not just what the U.S. spent on fighting in Iraq and Afghanistan, but also what it will spend on veterans' health care and benefits; equipment refurbishment; future commitments made to the Iraqi and Afghan governments the U.S. sponsors; and the repayment of the debt incurred by financing the wars through foreign borrowing. Notably, by Bilmes' framework, the real costs of the wars will only manifest long after the troops have come home.

She's also under-counting. The shadow wars in Yemen, Pakistan, east Africa and north-central Africa will not cost nearly as much as the Army-intensive wars of Iraq or Afghanistan. But they'll still cost something, either through leased infrastructure to base aircraft and special-operations forces; political commitments to host governments; support to allied war efforts; and some personnel costs. All these wars have the same wellspring as Iraq and Afghanistan: U.S. overreaction to terrorism.

"One of the most significant challenges to future US national security policy will not originate from any external threat," Bilmes writes. "Rather it is simply coping with the legacy of the conflicts we have already fought in Iraq and Afghanistan."

Money, ultimately, is power. In context, it would take a nuclear strike on the United States to inflict the kind of economic damage that the wars have reaped. The only nations capable of inflicting such damage are disinclined toward doing so; and no non-state actor will plausibly obtain the capability to match such a threat. All of that damage is the result not of what bin Laden or Saddam Hussein or the insurgencies that began in their wake did to America, but because of how American strategiests chose to respond. As Radiohead once sang, you do it to yourself, and that's why it really hurts.

Bin Laden didn't get what he wanted. The U.S. is not bankrupt. Any reduction of its political commitments to the Middle East, bin Laden's strategic goal, occur on the margins, and should Iran disrupt the world oil supply – the anchor of the U.S. commitment to the region – they'll quickly reverse. In wars, negative-sum outcomes are possible.

But the U.S. now has to labor under the economic strain of war costs that the wars' architects did not consider. They have introduced a cost-benefit calculation to U.S. involvement in future conflicts, like Syria's, that bellicose legislators find obnoxious or inconvenient. The lessons of Iraq and Afghanistan will be debated forever, but there's a rule of thumb that they immediately provide: ask for the bill before ordering the meal.