"Terrorism is, in the most general sense, the systematic use of terror"
Bombs, guns and bacteria aren't really necessary; a bomb threat a day phoned in to London Underground or City Airport will disrupt peoples daily lives far more effectively than actually blowing something up.
11/9 galvanised US opinion allowing an erosion of everyone's civil liberties, the invasion of two countries and the torture countless people.
A constant wearing down of the populous through loss of freedom, income and could be far more effective. The only challenge to the would e terrorist (group) is establishing credibility while evading capture Theodore Kaczynski (Unabomber) was active from 1978-95 and was only captured after his brother recognized Ted's style of writing and beliefs from the manifesto, and tipped off the FBI