Terrorist groups vary greatly in terms of their longevity, structure, and evolution. The longest-lived groups in recent years include Euskadi ta Askatasuna (ETA), established in 1959; Revolutionary Armed Forces of Colombia (FARC), established in 1964; National Liberation Army of Colombia (ELN), established in 1964; Palestine Liberation Organization (PLO), established in 1964; Popular Front for the Liberation of Palestine (PFLP), established in 1967; Provisional Irish Republican Army (PIRA), established in 1969; and the Moro National Liberation Front (MNLF), established in 1970. Most terrorist groups are more ephemeral; the median duration of transnational groups is between 1.7 and 3 years (Blomberg, Engel, and Sawyer, 2010, p. 319). In fact, there is a preponderance of terrorist groups that strike only once, called “one-hit wonders” by Blomberg, Engel, and Sawyer (2010). Terrorist groups are generally driven by one of four orientations: left-wing, nationalist/separatist (henceforth, nationalist), religious, and right-wing. Many single-issue groups – for example, animal rights or environmental protection – can be placed into one of these four categories. Nationalist, left-wing, and religious terrorist groups correspond to the second, third, and fourth waves of modern terrorism, respectively.

Terrorist groups have used many alternative organizational structures. At first, terrorist groups tended to be hierarchical organizations with a clear chain of command and many linkages among members. Terrorists discovered that hierarchical, densely linked structures are vulnerable, because the arrests of one or two terrorists could result in the entire group being compromised. A vivid example of this vulnerability was the Italian Red Brigades. In the aftermath of the General Dozier kidnapping in 1982, Red Brigades member Antonio Savasta turned state’s evidence, which led to the arrests of 200 suspected group members (Mickolus, Sandler, and Murdock, 1989, vol. 1, p. 237). In fact, the bulk of the Red Brigades was captured and tried. Although sporadic terrorist incidents claimed by the Red Brigades occurred in subsequent years, the group ceased to be much of a threat after the massive arrests.