War is many things, and one of those things is expensive. A new academic study shows just how expensive the war on terror has been for the United States over the past 18 years, placing the total taxpayer cost at $6.4 trillion.

In order to tabulate the total cost of post-9/11 anti-terror operations since late 2001, the report — which was published last week by the Watson Institute of International and Public Affairs at Brown University — takes into account not only the costs of military operations in Iraq, Afghanistan, and Syria, but also military operations in other countries, costs incurred by the Department of Homeland Security and the public cost of taking care of the medical and disability issues faced by post-9/11 veterans.

"The mission of the post-9/11 wars, as originally defined, was to defend the United States against future terrorist threats from al Qaeda and affiliated organizations.Since 2001, the wars have expanded from the fighting in Afghanistan, to wars and smaller operations elsewhere, in more than 80 countries — becoming a truly 'global war on terror,'" the summary explains. "Further, the Department of Homeland Security was created in part to coordinate the defense of the homeland against terrorist attacks."

To put that $6.4 trillion figure in context, that is 103,330,803 times the annual U.S. median household income of $61,937, meaning that it would take the annual median salaries of more than 100 million American households to cover the cost of the United State's Middle East and Asia involvement over the past 18 years as estimated by the report.

Furthermore, this figure is $2 trillion more than the entire U.S. federal government spent in fiscal year 2019, which was $4.44 trillion according to the Treasury Department.

A similar cost estimate put out by the Watson Institute in November 2018 estimated a total cost of $5.9 trillion, representing an estimated $500 billion price tag for the previous fiscal year.

The report's summary also explains that costs to American taxpayers resulting form the war on terror would keep adding up even if the U.S. were to cease its military operations in the Middle East and elsewhere by the end of this fiscal year. This is because, it says, of the ongoing cost of veterans' care and also because "the increases in the Pentagon base budget associated with the wars are likely to remain, inflating the military budget over the long run."

Meanwhile, the United States' national debt continues to grow with no end in sight, topping $23 trillion for the first time in history at the beginning of November. Furthermore, the budget deficit for October grew 34 percent over the previous year, which puts the country on course to hit a trillion-dollar deficit this fiscal year.