Terrorism reaches back to ancient Greece and has occurred throughout history. Terrorism by radicals (of both the left and right) and by nationalists became widespread after World War II. Since the late 20th cent. acts of terrorism have been associated with the Italian Red Brigades, the Irish Republican Army, the Palestine Liberation Organization, Peru's Shining Path, Sri Lanka's Liberation Tigers of Tamil Eelam, the Weathermen and some members of U.S. militia organizations, among many groups. Religiously inspired terrrorism has also occurred, such as that of extremist Christian opponents of abortion in the United States; of extremist Muslims associated with Hamas, Osama bin Laden's Al Qaeda, and other organizations; of extremist Sikhs in India; and of Japan's Aum Shinrikyo, who released nerve gas in Tokyo's subway system (1995).

In 1999 the UN Security Council unanimously called for better international cooperation in fighting terrorism and asked governments not to aid terrorists. The Sept. 11, 2001, attacks by Al Qaeda on the World Trade Center and the Pentagon—the most devastating terrorist attacks in history—prompted calls by U.S. political leaders for a world war on terrorism. Although the U.S. effort to destroy Al Qaeda and overthrow the Afghani government that hosted it was initially successful, terrorism is not a movement but a tactic used by a wide variety of groups, some of which are regarded (and supported) as freedom fighters in various countries or by various peoples. So-called state-sponsored terrorism, in which governments provide support or protection to terrorist groups that carry out proxy attacks against other countries, also complicates international efforts to end terror attacks, but financial sanctions have been placed by many countries on organizations that directly or indirectly support terrorists. The 2001 bioterror attacks in which anthrax spores were mailed to various U.S. media and government offices may not be linked to the events of September 11, but they raised specter of biological and chemical terrorism and revealed the difficulty of dealing with such attacks.