Combating crime mainly relies on deterrence through punishment of criminals who recognize that there is a chance of being apprehended and convicted-the chances are greater for more serious crimes. If convicted, they can expect imprisonment or other punishments- again, punishments are generally more severe for more serious crimes. Apprehension and punishment reduce the gain from crimes; in this way, it deters others from criminal activities.
Individuals can also be punished simply for planning or intending to commit crimes. The evidence required to punish intent has to be convincing, but the standard is weaker for violent crimes, like plotting murder, since punishment after the crime does not do anything for those murdered. In addition, individuals who cannot be deterred are sometimes punished simply because it is considered likely that will commit crimes in the future. This is a major justification for forced hospitalization and psychological treatment of potentially violent and mentally unstable persons.
These arguments about intent apply much more strongly to preventive actions against terrorist organizations and rogue nations. The conventional approach to war in democratic states favors retaliation after attacks. This was the rationale for the Mutually Assured Destruction (MAD) doctrine during the height of the Cold War: the US was prepared to unleash devastating nuclear destruction against the Soviet Union if attacked with nuclear weapons, and visa versa for the Soviets. That worked, although there were several close calls, as during the Cuban crisis.
But this approach is no longer adequate to fight terrorist organizations, states that sponsor terrorism, and dictatorial states that want to destroy their enemies. For it is becoming increasingly possible for terrorist organizations and governments to unleash biological, chemical, or nuclear weapons that will cause massive destruction. Retaliation may be slow and difficult if terrorists are widely dispersed so that it is hard to generate sufficiently severe reprisals to discourage their attacks. Rogue governments also are more capable of using these weapons surreptitiously, so that it might be many obstacles to determining who was responsible if they chose not to admit their responsibility. It is already difficult to know which groups are responsible for terrorist acts except when they brag about them.
In addition, many state-sponsors of terrorism often prey on the zeal of individuals who are willing to kill themselves in promoting what they consider a higher cause. These suicide bombers clearly cannot be punished after they commit their acts (although their families could be) because they forfeit their lives while attempting to kill and injury others. One can try to raise the probability that they will fail-through barriers, walls, and other protective activities- but free societies are so vulnerable that these can never be strong enough deterrents.
The only really effective approach is to stop them before they engage in their attacks. This is accomplished by tracking them down and imprisoning or killing them based on evidence that they intend to engage in suicidal attacks. Those planning such acts can also be punished on the basis of intent.
The same argument applies to dictators who are willing to use weapons of mass destruction to attack their enemies when they do not care if many of their populations are killed and maimed by retaliation from other countries. Dictators, like Saddam Hussein, may also greatly underestimate the likelihood of massive responses because sycophants feed them bad information, or they believe that democratic victimized states will be reluctant to make swift and decisive responses.
Admittedly, the evidence is usually more imperfect when trying to prevent attacks than when responding to attacks. Mistakes will be made, and the evidence of intent must be analyzed carefully. But criminals are convicted too on less than 100% certain evidence. As Posner says in his commentary, it is necessary to consider probabilities, not certainties.
Moreover, the degree of certainty required before preventive actions are justified has been considerably reduced below what it was in the past because the destructive power of weaponry has enormously increased. Perhaps most worrisome, the power of weapons continues to grow, and to become more easily accessible. Critics of preventive wars and other preventive actions against rogue states and terrorist groups ignore these major changes in weaponry and their availability. Democratic governments have to recognize that they no longer have the luxury of waiting to respond until they are attacked.