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Costs to U.S. taxpayers of the wars in Iraq and Afghanistan will run between four and six trillion dollars, making them the most expensive conflicts in U.S. history, according to a new report by a prominent Harvard University researcher.

While Washington has already spent close to two trillion dollars in direct costs related to its military campaigns in the two countries, that total “represents only a fraction of the total war costs”, according to the report by former Bill Clinton administration official Linda Bilmes.

“The single largest accrued liability of the wars in Iraq and Afghanistan is the cost of providing medical care and disability benefits to war veterans,” she wrote in the 21-page report, ‘The Financial Legacy of Iraq and Afghanistan: How Wartime Spending Decisions Will Constrain Future National Security Budgets’.

Bilmes, who since 2008 has co-authored a number of analyses on war costs with the World Bank’s former chief economist, Joseph Stiglitz, noted that more than half of the more than 1.5 million troops who have been discharged from active duty since 9/11 have received medical treatment at veterans’ hospitals and have been granted benefits for the rest of their lives. More than 253,000 troops have suffered a traumatic brain injury.

Additional costs include the replacement and repair of equipment – which wears out at an estimated six times the peace-time rate – and the accumulation of interest on money borrowed by the Treasury to finance the wars since the nearly two trillion dollars in war costs were not subject to the normal budgetary process.

So far, Washington has paid some 260 billion dollars in interest charged on war-related borrowing, but the “potential interest cost of the U.S. war debt reaches into the trillions,” according to the report.