Last month we observed the solemn 18th anniversary of the terrorist attacks of Sept. 11, 2001. We have made significant strides since then in securing our nation against large-scale terrorist attacks from foreign groups like al-Qaida. We have established a Department of Homeland Security; strengthened aviation and infrastructure protections; constructed an integrated intelligence apparatus; and struck strong blows against terrorist enclaves overseas.
This work must continue as ISIS, al-Qaida and other extreme violent Islamist organizations seek to regroup. But we must recognize that in the past two decades, the threats of terrorism and mass violence have metastasized. We have seen the emergence of new strains of ideologically motivated terrorism and targeted mass violence, much of which is launched by domestic actors.