I think there is a very high probability of a terrorist attack on the USA involving either a nuclear or a biological weapon. In fact, I think it's only a matter of time. Virtually inevitable.

The trouble is, the main cause of that attack will be: the reaction of some angry 3rd World people to the past few decades of US foreign policy and especially to military actions taken since 911.

If it happens, it will most likely be a clandestine attack by a secret cell, not an action by a national government. But the USA will find a national government to blame for it...most likely one that is parked on some valuable strategic real estate. And that will bring forth further tragedy and further retaliation against the USA.

The USA, like Imperial Rome or Soviet Russia or any other conquering empire in history, is the author of its own misfortunes.