Argument from ignorance (because we do not know X is false, therefore it is true) is typically thought to be a fallacious form of argument. Yet it is precisely this argument that is used to argue that pre-emptive action should be taken against unknown but risky threats. Such an argument is a staple of discourse about terrorism, which typically involves stealth. In some cases, the argument from ignorance can be completely reasonable. This essay compares the use of the argument form by National Security Adviser Condoleezza Rice and Secretary of State Colin Powell to justify war against Iraq. Powell’s approach is defended whereas Rice’s is criticized. This essay originally was presented at the 14th biennial National Communication Association/American Forensic Association Summer Conference on Argumentation, held at Alta, Utah in 2005. It is reprinted here from the conference volume, Engaging Argument (Patricia Riley, Ed.), pp. 29–35 (Washington: National Communication Association, 2006) and is reprinted with permission of the National Communication Association.