More than a trillion dollars has been appropriated since September 11, 2001 for U.S. military operations in Iraq, Afghanistan and elsewhere. This makes the “war on terrorism” the most costly of any military engagement in U.S. history in absolute terms or, if correcting for inflation, the second most expensive U.S. military action after World War II.
A newly updated report from the Congressional Research Service estimated the financial costs of major U.S. wars from the American Revolution ($2.4 billion in FY 2011 dollars) to World War I ($334 billion) to World War II ($4.1 trillion) to the second Iraq war ($784 billion) and the war in Afghanistan ($321 billion). CRS provided its estimates in current year dollars (i.e. the year they were spent) and in constant year dollars (adjusted for inflation), and as a percentage of gross domestic product. Many caveats apply to these figures, which are spelled out in the CRS report.
In constant dollars, World War II is still the most expensive of all U.S. wars, having consumed a massive 35.8% of GDP at its height and having cost $4.1 trillion in FY2011 dollars. See “Costs of Major U.S. Wars,” June 29, 2010.