What Is Terrorism?

On September 11, 2001, nearly three thousand Americans died when terrorists in hijacked planes attacked major government and business sites in New York and Washington D.C, causing the deadliest terror attack in history. In July 2005, nearly sixty people in the United Kingdom died at the hands of terrorists who bombed busy trains targeting unsuspecting commuters in London. And in 2015, al-Shabaab militants in central Kenya killed more than seven hundred students at a university in Garissa. What drives such deadly violence around the world? And how are those threats changing and evolving to shape the concerns most relevant to policymakers today?

Those attacks are all examples of terrorism, violent acts distinct from other classifications of violence such as murder and casualties from war. Terrorism is, unfortunately, a longstanding concern for governments all over the world. Although some terrorist threats have persisted for years or even decades, others have evolved more recently in relation to escalating political and economic challenges and new forms of technology.

Terrorism is a loaded term, one that stirs both fear and confusion. Public debate in the wake of attacks can make it difficult to grasp the nature of threats and to assess responses. But understanding terrorism is important, because policies enacted in the name of national security affect a wide range of issues, such as security and freedom on mobile phone apps, policies surrounding the admission of refugees, and distribution of foreign aid.

Defining Terrorism

Definitions of terrorism differ around the world. Policymakers consider differences depending on whether terrorist threats originate inside the country (domestic terrorism) or outside of the country (international terrorism). Usually key to all of these definitions is the political or ideological motivation behind violence attacks.

In the United States, the Federal Bureau of Investigation (FBI) defines international terrorism as "Violent, criminal acts committed by individuals and/or groups to further ideological goals stemming from domestic influences, such as those of a political, religious, social, racial, or environmental nature.”

Throughout history, terrorists all over the world have advanced diverse political agendas. Despite the prominence of terrorist threats in U.S. public debate, especially since the deadly 9/11 attacks in the United States, most terrorism threats aren’t actually aimed at the United States. The vast majority of the world’s recent terrorism-related attacks have occurred in Afghanistan, Burkina Faso, Mali, Pakistan, and Somalia. And, in more recent years, while international terror threats still remain a critical concern for the United States, the threat of domestic terrorism has been a growing concern for U.S. national security experts.

Explore CFR Education resources on terrorism to