After 18 years, two wars, multiple conflicts, and billions of dollars spent, the United States is still grappling with the threat posed by international terrorism, but questions linger as to whether policymakers truly understand their adversary.

When 19 terrorists hijacked and flew airliners into the twin towers of New York’s World Trade Center, as well as the Pentagon and a field in Pennsylvania, killing nearly 3,000 people, the U.S. dedicated its defense, intelligence, and national security infrastructure toward learning about the group responsible. Those resources have been effective at preventing similar large-scale attacks, but al Qaeda and other jihadist terrorist groups have found ways to persist.