U.S. war operations around the world for terrorism have cost more than $5.9 trillion since the Sept. 11 terrorist attacks, according to a study by the Watson Institute for International and Public Affairs at Brown University.

The analysis said that figure will rise to $7 trillion if present military conflicts continue into the 2020s.

"The Pentagon currently projects $80 billion in Overseas Contingency Operations (OCO) spending through FY2023. Even if the wars are ended by 2023, the US would still be on track to spend an additional $808 billion to total at least $6.7 trillion, not including future interest costs," the report said.