Terrorism is pretty much what Emissionguy says.
I don't think it's a desperate move, but one that has become a legitimate form of warfare nowadays and when I say legitimate, I mean that from the point of view of the parties that utilise terror in order to further their cause.
Sadly, I don't think it's a case of overcrowding either, but a means of causing great pain and suffering to a lot of people without needing a full-scale army to achieve their goals as generally, their enemies are thousands of miles away.
Look at 9/11. There were very few people who were actual terrorists, yet thousands if not millions were affected by it. The London tube bombings were another case in point.
I do agree that I don't think it can be stopped though.
There will always be someone, somewhere who thinks that intimidation and terrorism will get them what they want.