Social dynamics of terrorist groups
A lot of people worry that fighting Islamic terrorism can only encourage more people to become terrorists. The typical argument is that by fighting in Iraq, we are only creating a larger population of terrorists. However, there is a difference in scale. For terrorists to train in groups requires implicit state support. In a post at Global Guerrillas, John Robb, focusing on the network aspect of terrorism (via Belmont Club), looks at the social dynamics of terrorist groups and the impact on their potential size.
you can have small, operationally secure terrorist groups, but you can't have large, operationally secure cells without a state sponsor.
I think this is right. Without state support (as was the case Afghanistan, and is the case in Iran or Syria), it's very hard for large-group training to take place.