At 8:46 a.m. ET on Tuesday, September 11, 2001, American Airlines Flight 11 crashed into the north tower of the World Trade Center—the first of four plane crashes that morning—and nothing was ever the same again. This year marks the twentieth anniversary of the 9/11 attacks, and in this episode of Big World, SIS professor Josh Rovner joins us to discuss the national security legacy of 9/11. Professor Rovner shares where he was when the first plane hit the north tower (2:03), explains what then-president George W. Bush called the “War on Terror” in response to the attacks (3:00), and discusses some of the immediate impacts of 9/11 on national security (5:58). He also describes the long-term changes to national security measures after 9/11 that continue to impact Americans today (8:41) and how the legacy of 9/11 and the War on Terror impacted the overall US defense apparatus (11:10). What was achieved during the 20-year mission in Afghanistan, which was America's longest running war (17:56)? How does the US approach counterterrorism now that troops have been withdrawn from Afghanistan and the Taliban is in control (21:48)? Professor Rovner answers these questions and explains how the priorities of the Department of Defense and the Department of Homeland Security have been affected by 9/11 (25:07); he also shares an unforeseen legacy of the attacks that might surprise people (27:57). During our “Take Five” segment, Professor Rovner details the advice he would give to the current secretary of state, secretary of defense, and director of national intelligence as the Biden administration implements its approach to counterterrorism (13:38).