To develop its national security strategy, the U.S. government typically formulates a set of agreed definitions and concepts about the current threat environment to help delineate top priorities. Once established, these impose clarity on mission spaces, shape the delegation of tasks and operations, justify budgets and the distribution of resources and personnel, and influence organizational structures. The notion of weapons of mass destruction (WMD) has long served as one of these key defining concepts that shape U.S. national security strategy. The two most common characteristics used to define WMD are mass casualties and mass destruction, as indicated by the U.S. Department of Defense’s definition. These characteristics were intended to clarify why the U.S. places top priority on WMD in its national security strategy; they are capable of significant damage in terms of destruction, casualties, and strategic impact.
Within the U.S. context, concepts about the threat environment tend to evolve incrementally with changes in administration and the emergence of new technologies and threats, or specific events that dramatically alter threat perceptions. However, the longer certain threat perceptions survive the political wrangling of opposing interests, the more likely they evolve into threat paradigms—i.e., patterns or standard models of behavior, against which any new ideas about emerging threats are thoroughly tested. In such cases, a major catalyzing event must occur to produce dramatic change and reorganization of national security priorities.
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 DoD Dictionary of Military and Associated Terms, June 2019.
 See W. Seth Carus, Defining “Weapons of Mass Destruction”, Occasional Paper No. 8, Washington D.C.: National Defense University, 2012.