Stopping terrorists' ability to enact terror
The four elements of counterterrorism policy -- attacking the roots, capabilities, intentions, and defenses of terrorist groups -- the most important in my mind is capabilities. While not as easy to enact as others, there is something to be said for effectively staving off a terrorist's capability to destroy and kill.
Capabilities are the terrorist's abilities to carry out attacks or to conduct them effectively. This is an incredibly difficult facet of terrorism to address but incredibly effective. This work involves a variety of intelligence, legal, and other counterterrorist instruments. An example of a terror cell being dealt a crushing blow was the Tupac Amaru Revolutionary Movement's (MRTA) 1997 overthrow in Lima, Peru. The MRTA had seized the Japanese ambassadorial residence in Lima four months previously and had kept hostages for that time. A rescue was mounted and all but one of the seventy-two hostages were freed. This raid also crippled the MRTA's capability to conduct further terrorism through cell casualties and law enforcement's disruption of current and future MRTA projects.
Denying capabilities to terrorist groups has been quite effective in the past, as acts of disruption have caused cells to regroup and try to find other ways to enact their carnage. However, it is always clear that terrorists do not need much along the lines of opprotunity to make things happen. Timothy McVeigh employed just enough deception and skill to get his bombing to occur; though it was not enough to prevent his capture, the damage had already been done and his statement had been made. So while not foolproof, denying the capability of terror groups to act is an essential part of the overall counterterrorism strategy.