The Bush administration's 2002 National Security Strategy touched off a vigorous debate in the United States and abroad, which intensified with the war in Iraq, over what appeared to be a novel, broad assertion of the right to use force to prevent latent threats from emerging, particularly threats associated with terrorism and nuclear, biological or chemical weapons. But the use of preventive force - and the debate over its legality and wisdom - predate the Bush administration's post-11 September 2001 strategy. A careful examination of the history, rationale, costs and benefits of using preventive force suggests that, while rare, preventive force has a legitimate role to play in tackling some of the most dangerous security problems facing the United States and the wider international community.