Non-violence cannot defeat terrorism but then nothing can 'defeat' terrorism. Terrorism is a tactic an under certain conditions it becomes a seemingly logical tactic for it's proponents. Looking at all terrorists rather than narrowing in on the Islamic Extremists shows this. Terrorists exist in many places because terrorism is one of the last resorts in a conflict. If your enemy has nukes, bombers, drones, helicopters, missiles, aircraft carriers, satellite imaging, tanks and special forces while you have an AK-47 and an online bomb recipe terrorism seems a lot more appealing than conventional warfare.

The goal of terrorism is to cause sufficient fear in the general public to force a government to take or not take certain actions. Non-violence would not succeed in preventing terrorism but neither would violence. At this point in time terrorism cannot be defeated and as weaponry becomes more powerful and requires less personal risk it is likely to become a more serious threat. Anyone can buy a drone now, it's only a matter of time before someone starts using drone bombs or manages to use some of the more dangerous weapons that the US sells to unstable dictators like Saddam Hussein or terrorists like Osama bin Laden (or before they arm someone like ISIS enough for it to be a more serious threat).