Book excerpt: The prospect of chemical, biological, radiological, and/or nuclear (CBRN) terrorism is recognized by the United States government as an acute security challenge. Particularly following the tragedy of September 11, 2001, but also for several years prior, senior U.S. officials and official government reports have underscored the likelihood, over time, of terrorist organizations coming into possession of such unconventional materials, and the prospect of their use against the United States homeland, U.S. forward-deployed forces, or U.S. friends and allies. Toward the end of the last century, this concern was heightened, among other events, by the Japanese cult Aum Shinrikyo's 1995 use of sarin in the Tokyo subway. The combination of increasing availability of technology and expertise, a perceived mass-casualty motive structure for particular terrorist organizations, the impending end of a millennium, a spate of conventional attacks against U.S. assets World Trade Center, 1993; Oklahoma City Federal Building, 1995; American embassies in Tanzania and Kenya, 1998; and the U.S.S. Cole, 2000 and both the widespread suspicion of terrorists seeking CBRN weapons and the actual sub-national employment of a chemical agent all contributed to this general assessment.