The attacks of September 2001 transformed national security in the United States. Legacies of the past decade—two major military campaigns, an unprecedented series of policy reforms and innovations across government, and a broad mandate of aggressive counterterrorism strategy—have influenced U.S. policy and strategy in unique and sometimes immeasurable ways. Following the death of Osama bin Laden and with ongoing instability in Iraq and Afghanistan, how should we view America’s response to 9/11? What have we learned about the limits of containing and preventing terrorism? In a new era of constrained resources and persistent international threats, how will America face the next decade of security challenges?