Why?
I understand the tragedy of 9/11. With that in mind, it is a mistake is to treat terrorism as an existential threat to the state. Perhaps someone with a more formal background in history can speak to this, but I do not think there is an example of a nation falling because of terrorism.
Point: there is a widespread purported belief in western democracies, particularly the United States and Great Britain, that one or more terrorist attacks could cause institutional collapse. This is ridiculous, however well-intentioned it might have started out as being.