As of August 2017, the Department of Defense (DoD) had obligated $1.474 trillion for war-related costs since September 11, 2001. DoD updated its official cost report last month. See Cost of War Update as of August 31, 2017.
Average monthly spending in 2017 was $3.9 billion, up from an average of $3.5 billion in 2016, the report said.
The total post-9/11 spending figure includes $83 billion in classified appropriations, not including non-DoD classified expenditures (e.g. CIA spending).
The reported costs of war are highly dependent on the definition of the term. DoD’s total figure, which does not include many kinds of indirect costs, is substantially lower than estimates by other analysts such as the Watson Institute at Brown University, which placed the total figure at $5.6 trillion as of November 2017.
The DoD report also understates the number of US troops deployed in Afghanistan, Iraq and Syria.