The United States may have stronger terror-fighting capabilities than before the 9/11 attacks, but experts worry about the ability to gather intelligence in Afghanistan without any troops in the country.

The U.S. launched the war in Afghanistan in response to the attacks on Sept. 11, 2001, when 19 al Qaeda terrorists hijacked and crashed planes into the World Trade Center buildings, the Pentagon, and a field near Shanksville, Pennsylvania, killing nearly 3,000 people. Troops remained for nearly 20 years to stop the country from being a haven for terrorist organizations.