By Patrick Appel

A new RAND report tackles the question:

The comprehensive study analyzes 648 terrorist groups that existed between 1968 and 2006, drawing from a terrorism database maintained by RAND and the Memorial Institute for the Prevention of Terrorism. The most common way that terrorist groups end -- 43 percent -- was via a transition to the political process. However, the possibility of a political solution is more likely if the group has narrow goals, rather than a broad, sweeping agenda like al Qaida possesses.

The second most common way that terrorist groups end -- 40 percent -- was through police and intelligence services either apprehending or killing the key leaders of these groups. Policing is especially effective in dealing with terrorists because police have a permanent presence in cities that enables them to efficiently gather information, Jones said.

Military force was effective in only 7 percent of the cases examined; in most instances, military force is too blunt an instrument to be successful against terrorist groups, although it can be useful for quelling insurgencies in which the terrorist groups are large, well-armed and well-organized, according to researchers. In a number of cases, the groups end because they become splintered, with members joining other groups or forming new factions. Terrorist groups achieved victory in only 10 percent of the cases studied.