The continued proliferation of weapons of mass destruction (WMD) represents the most serious threat to U.S. national security and an enormous challenge for the entire international community. In the hands of rogue states, failing states, or substate terrorist groups, these weapons threaten not only U.S. forces, friends, and allies abroad, but also the U.S. homeland. Rogue states armed with WMD threaten the security of regions that are vital to the United States and raise the costs and risks of U.S. military operations intended to protect those interests. Forming international coalitions to defend shared interests is also more difficult in the shadow of the rogue state WMD threat. WMD in the hands of regional rivals exacerbate long-standing conflicts and increase the potential for escalation and conflict. Finally, terrorist interest in WMD and the threat of terrorist use of such weapons appear to be growing.