The following is the Feb. 26, 2019 Congressional Research Service report, Afghanistan: Background and U.S. Policy In Brief.
From the report
Afghanistan has been a central U.S. foreign policy concern since 2001, when the United States, in response to the terrorist attacks of September 11, 2001, led a military campaign against Al Qaeda and the Taliban government that harbored and supported it. In the intervening 17 years, the United States has suffered around 2,400 fatalities in Afghanistan...
https://news.usni.org/2019/02/27/report-congress-u-s-policy-toward-afghanistan