The U.S. began its war on terrorism following the attacks of Sept. 11, 2001. The focus began on the group Al Qaeda but has since expanded to include the Islamic State.

Considering the non-traditional challenges of fighting terrorist groups instead of more traditional nation-centered adversaries, some have questioned whether the war on terror is being fought correctly and whether it will ever end.

One person who has looked into the topic extensively and wrote about it recently is Katherine Zimmerman, research fellow at the American Enterprise Institute and the research manager for AEI’s Critical Threats Project.