This article examines theories of practical and strategic constraints that collectively explain why so few terrorist groups in history have crossed (or attempted to cross) the WMD threshold. From this analysis, it becomes clear that a terrorist group's deliberations about WMD can be influenced (positively or negatively) by a variety of factors. Our projections of the future WMD terrorism threat must therefore account for changes in the kinds of practical and strategic constraints that could

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... to an increased willingness and/or capability of a group to pursue these kinds of weapons. Further, there are ways in which governments can influence a terrorist group's decision-making and thus have a direct impact on the future evolution of the WMD terrorism threat.