Threat assessment for weapons of mass destruction is a complex task, requiring many assumptions. As a general rule, weapons of mass destruction are expensive, complex, and difficult-to-use weapons. It is not likely that any current terrorist group has the capability to strike the United States with a weapon capable of producing millions of casualties. Smaller-scale attacks with weapons of mass destruction, however, may result in significant disruption from social and psychologic changes, even though actual casualty rates would probably be quite low. It is, however, highly unlikely that any terrorist attack on the United States could completely undermine national security or threaten the survival of the United States as a nation.