"Al-Qa’ida’s operations were especially deadly even in comparison to other notorious, long term terrorist organizations:
ETA, the Basque nationalist terrorist group in Spain, has been responsible for approximately 820 deaths from 1972 to 2008.
The Irish Republican Army (IRA) was responsible for 1829 fatalities dating back to 1970—less than half of the number of people killed by al-Qa’ida.
The Revolutionary Armed Forces of Colombia (FARC) has been responsible for 4,835 terrorism fatalities in its history. While FARC has imposed this bloodshed over the course of more than 30 years, Al-Qa’ida’s 4,299 deaths were concentrated in just a 10-year period.
Since 1998, there have been 408 incidents of mass-casualty terrorism—single events in which more than 25 people were killed. Al-Qa’ida was responsible for 16 mass-casualty terrorism attacks—more than any other group during this same period.
Al-Qa’ida has also become a crucial “node” of a network of deadly terrorist organizations—some created in the hopes of replicating al-Qa’ida, others aligning with al-Qa’ida for ideological or practical reasons. Research by Victor Asal and R. Karl Rethemeyer at the University of Albany (SUNY) has identified 33 different terrorist organizations with direct links and alliances to al-Qa’ida."
START background report, May 2011