I am impressed by the very high quality of most of the comments and by the lively exchange among the commenters. If our blog can stimulate such exchanges, it is a success already. I will limit my response to a few recurrent themes in the comments. One, a minor point, has to do with my having said, all too cryptically, that "the essence of self-defense is striking the first blow against your assailant." Readers thought "self-defense" should be replaced by "offense." I was thinking of the defense of self-defense in criminal law: if someone is about to stab you and you could prevent him from doing so by shooting him but you decide not to do so because you would be guilty of murder, then you have misunderstood what self-defense means in the law. A major theme in the comments is that it is impossible to assign a numerical probability to an adversary's attack, unless the attack is imminent. That is true. No one could have said in 1936 that if Hitler was allowed to reoccupy the Rhineland, there was a .__ probability that he would eventually attack France. However, we frequently have to act under conditions of profound uncertainty. It would be paralyzing to suggest that we should never act unless we can quantify the expected benefits and costs of our acts (there would be very few marriages under this approach). And readers who doubt that cost-benefit analysis can be applied to matters of life and death, such as war, should consider that lives are on both sides of the balance. If a preventive war that killed 10,000 people could prevent a nuclear attack on the United States that would kill 10 million people, such a war would in my opinion be justified. (I wonder how many readers actually disagree.) My 1936 Rhineland example was misleading in the following respect: horrible as World War II was, the 50 million killed and the untold destruction and immiseration caused might be dwarfed by what a small nation or even a terrorist gang--perhaps even a biological Unabomber--could do, if not today, then in the near future. It is the unprecedented dangers created by modern technology that require reconsideration of the traditional prejudice against preventive war. In favor of a categorical rule against preventive war, some commenters argued that such a rule is necessary to prevent a "slippery slope" that would end with Canada and Mexico invading the U.S. out of fear that our invasion of Iraq indicated that we might be trying to conquer the world. In other words, the argument goes, the existence of such a rule reduces the likelihood of all war. But this is very unlikely. Nations will not obey rules unless it is in the national self-interest to do so. Whether or not there is a rule against preventive war, no nation will launch such a war unless it thinks it necessary for national survival or some equivalent good, and if it does, it will not be inhibited by a rule. I thus disagree with those readers who think that "legitimacy" plays a big role in international affairs. I do think that reciprocity plays a big role. If our waging a preventive war created a risk of others' waging a preventive war against us, that would certainly be a powerful argument against preventive war. Some readers suggest that my posting was intended to advance a hidden political agenda that includes defending the war in Iraq. Not at all; and I mentioned the failure to find weapons of mass destruction as an example of the uncertainty that plagues any decision to launch a preventive war rather than to wait for one's enemy to strike the first blow. But that uncertainty is just one of the factors that must be considered in deciding for or against a preventive war. Although many of the comments criticize the war in Iraq, I do not recall a single criticism of the war in Kosovo. Yet that was not a defensive war. Milosevic's Serbia was not threatening other nations; Kosovo was a province of Serbia. It was not a preventive war; my reason for mentioning is that it shows that wars can be justified without being defensive. The case for preventive war must be debated on its merits rather than rejected outright on the ground that any war that is not defensive is aggressive and therefore "illegitimate."