Most terrorism groups never last long enough to achieve their objectives, at least 90% of terrorist groups last less than a year, and even those that do are rarely successful.  Part of the nature of this lack of success is because terrorist groups tend to have absolutist goals rather than easily achievable ones. For example, Al Qaeda’s stated goal is to establish a new Islamic caliphate. Determining a group’s success depends upon what metrics you judge the group on. If you look at tactical success than Al Qaeda is enormously successful, it killed thousands of people during the 9/11 attack and has had other successful attacks against American and Arab state targets. However, terrorist groups are political actors, not psychopaths, so judging groups by how many people they kill or property they damage misunderstands the nature of terrorism. The clearest way to judge the success of a terrorist group is measuring their outcomes versus their stated objectives.
Of all the types of terrorist groups, anti-colonial ones have had the most success. There are several reasons for this, such as limited goals, support of the target population, and an adversary that is not firmly rooted within the target population and does not have vital interests threatened. Two examples of groups that used terrorism against a colonial power that were successful include the Irgun in Israel and the Irish Republican Army in Ireland. These successes happened after WWII following a general trend of decolonization. Even the success of these groups has to be qualified, Irgun wanted to gain control over the entire Palestine Mandate, but had to accept a partition, and the IRA fell short of its goal of retaking Ireland in Northern Ireland, where a majority still support the British. These groups used terrorist tactics, but eschewed the title ‘terrorist’ and instead used the language of freedom fighters to demonstrate their legitimacy over the foreign occupying force. These groups were also able to use this legitimacy to gain financial support from Diaspora groups which meant that they had less need to be involved in criminal fundraising. Despite these groups success, it is questionable how much of the success depended on terrorist tactics and how much was based on the groups willingness to turn to political methods to settle their grievances. One of the most successful moments for the IRA was their prison hunger strike that didn’t involve any violence, and one of their least successful moments was the terrorist 1978 restaurant bombing that killed 12 civilians and resulted in a public outcry against the group. Even when terrorist groups achieve their goals, it is not usually exclusively because of their use of terrorism as a tactic, but because of a combination of terrorism and political actions, or terrorism combined with guerilla warfare.
 Bruce Hoffman, Inside Terrorism, (New York: Columbia University Press, 2006),241.
 David C. Rapoport, “Modern Terror: The Four Waves,” in Attacking Terrorism: Elements of a Grand Strategy, ed. Audrey Cronin and J. Ludes (Washington, D.C.: Georgetown Univ. Press, 2004), 53-54.
 1978: Belfast bomb suspects rounded up (accessed May 2, 2014); available from http://news.bbc.co.uk/onthisday/hi/dates/stories/february/18/newsid_2550000/2550869.stm