In building counterterrorism and homeland security capability, security planners at all levels face a variety of tradeoffs. Past attacks demonstrate that terrorist groups can attack targets many ways, and even among these proven threats, planners must weigh the costs and benefits of different security measures and allocate finite resources among them. For example, planners know that terrorists have staged attacks on commercial aviation with man-portable surface-to-air missiles, but they still must make the difficult decision as to whether the benefits of ways the nation could address this threat are sufficient to justify their cost. To protect both individual targets and...