[Disclaimer: This writing is not as "finished" as I would like it to be.]
terrorism - Terrorism is "a form of politically motivated action combining psychological (fear inducing) and physical (violent action) components carried out by individuals or small groups with the aim of inducing communities or states to meet the terrorists' demands" ("terrorism").
pre-emptive war - Pre-emptive war is war that is "waged in an attempt to repel or defeat an imminent offensive or invasion, or to gain a strategic advantage in an impending (usually unavoidable) war" ("Pre-emptive war").
wrong - Something is wrong if it is "not in accordance with a code, convention or set of rules" ("wrong"). That code is often morally or ethically-based.
Major Writing Assignment
When answering questions about the legitimacy of war, many people turn to the branch of philosophy known as just war theory. While there are various ways of organizing the components of just war theory, one could say (as does William V. O'Brien) that there are three main criteria that must be met for a war to be considered just: the war must be declared by "a competent authority for a public purpose," just cause must exist, and there must be a "right intention that aims at peace" (qtd. in White 419). The first and third criteria are certainly not free from debate or interpretation, but arguments on the justification of wars usually rise or fall on the second criterion. Just cause has some "formal" components within just war theory, but arguments made for just cause often fit no mold and are based on many different perspectives. This essay will step outside of a rigid just war theory structure to explore three ways in which pre-emptive war can satisfy just cause. Even in an age of terrorism, pre-emptive war is not always wrong because an imminent enemy attack is known, those planning the imminent attack have forfeited their right to life, and the total destruction can sometimes be minimized.
First, pre-emptive war is not always wrong because, by definition, the enemy attack is known to be imminent. Pre-emptive war is often confused with the concept of preventive war, in which there is only a potential for an enemy attack. For this reason, preventive war is often seen as a matter of aggression, while pre-emptive war is seen as an act of self-defense. It seems wrong to use aggression to justify an action as strong as war, while self-defense seems like a "nobler" justification.  Secondly, when the enemy attack is known to be imminent, pre-emptive war is justified because it is as if the war has already begun. By having definitively planned an attack, the enemy nation has declared war. In this sense, the pre-emptive act is not preceding war, but only the first destructive act. Before either of these points can be made, it must be understood that it is indeed possible to truly know that an enemy is going to attack. One shortcoming of traditional just war theory (or even less-formal arguments about the justification for war) is a focus on the factual conditions of the given situation to the exclusion of the epistemological conditions. Arguments on justification of war need to consider not only "what is the case and what is not the case," but also "what can be known, is reasonable to believe, and so on" (Dipert 36). For preemptive war to be justified, the "epistemic threshold" must be met, meaning that there can and must be "'objective certainty' about the enemy's intentions, bellicosity, and present and future military resources," not "merely a subjective certainty in feeling strongly about the extent of evidence for these factors" (Dipert 43). It should be conceded that situations in which this threshold is met are generally thought to be rare; however, the rarity of such situations is thought to be decreasing thanks to new technologies that have arguably increased the potential for certain knowledge of an attack. "In some cases, we will today be justified in believing that a nation will attack us in the future with greater epistemic justification than nations of the past had to believe that they had already been attacked," largely because of these advancements in technology (Dipert 41). While imminency of attack is a necessary condition for pre-emptive war to be justified, it must be shown why such an attack is worth impeding.
Using pre-emptive war in response to knowledge of an imminent attack is not always wrong because the right to life must be protected and the enemies planning the attack have forfeited their right to life by planning to steal that right from their target. To accept this argument, it must be first understood that the right to life is popularly thought to be given by God to every individual. Christians might look to the creation account in Genesis, in which God breathes life into humans (Genesis 2:7) or to the fifth commandment, "you shall not murder," (Exodus 20:13) in which they can see the inference that there is something about an individual's life that makes taking it away wrong.  Even non-biblical sources speak of the right to life as being God-given. The United States Declaration of Independence, for example, states that "all Men are created equal" and are "endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness [emphasis added]" ("Declaration of Independence"). For those who do not believe in a higher being who has granted this right, the preceding arguments do not hold much weight, but even these people still attach high value to the right to life and have developed irreligious systems for determining that depriving someone of this value is wrong and must be responded to. A popular example of such a system is retributive justice, as outlined by Immanuel Kant. Kant's work in philosophy centered on the concept of the categorical imperative, which says that a given action is right if the person acting would will all other persons to act by the same maxim. In terms of justice, this means that someone who is guilty of a crime must be punished appropriately. To not punish the guilty by applying their maxim to them would be to pervert the entire enterprise of justice and would undermine equality, Kant says, and "if justice and righteousness perish, human life would no longer have any value in the world" (qtd. in White 210).  Writing specifically on the issue of capital punishment, Kant says a person who kills another should be seen as also killing himself, as this maxim of committing murder must now be applied to him for justice to be upheld (qtd. in White 211). According to Kant's system, the death caused by pre-emptive war is justified because for justice to prevail, the maxim that life can be stolen, which has been adopted by the enemy nation, must be applied to that enemy nation. Terrorism, by its very nature, serves as a strong example of this forfeiture of right to life. On one hand, terrorism often includes the ending of others' lives. On the other hand, terrorists are often willing to forfeit their own right to life for their mission or cause. To use a recent historical example, those who saw the World Trade Center towers fall and were then bombarded with images and stories of terrorists and their motives, "knew" that these people were evil and that they were completely willing to disrespect their own right to life for the evil purpose of ending the lives of others. 
Thirdly, as it has been established that the basis for the evil of war is the destruction of the right to life, it can be said that any measure that would lessen the amount of destruction of life would be justified, so pre-emptive war is not always wrong because it can sometimes accomplish this objective. In this age of terrorism and advanced technology, nuclear weapons are a popular exemplification of just how utterly destructive war can be. Nuclear weapons are more destructive than any form of war weapon previously known because, rather than relying on a projectile or a "simple" explosion, they utilize the fission of radioactive materials. Aside from the devastating explosion, incredible destruction is caused by the radiation, electromagnetic and particulate, that occurs during and after the nuclear blast ("Nuclear Explosions"). The wide-spread damage to human life and health occurs in the form of blast injuries, burns, and radiation injuries ("Nuclear Explosions").  For pre-emptive war to be justified, not only must a nation in question possess weapons of mass destruction, but there must also be certain knowledge that these weapons will be used to attack and rob others of their right to life. So, if the destruction to life caused by an imminent attack, which could be almost incomprehensible when nuclear weapons are considered, could be lessened by a pre-emptive attack, pre-emptive war is justified. There also exists a more formal principle which attests to this argument, commonly called the "urgency principle." In his influential book Just and Unjust Wars, Michael Welzer distinguishes between legitimate and illegitimate pre-emptive strikes with the requirement of what he calls "sufficient threat," a "necessarily vague phrase" that includes three components, one of which is "a general situation in which waiting, or doing anything other than fighting, greatly magnifies the risk" (Walzer 81). Therefore, by the urgency principle, pre-emptive war is justified because acting preemptively can lessen the total destruction to life. Finally, historical examples show that the amount of total destruction can be minimized by a pre-emptive attack in some situations. Many people consider the Cuban Missile Crisis to be one such example, holding that the United States' pre-emptive action taken toward Cuba in the form of a blockade and other means generally regarded as acts of war prevented the much greater destruction that would have been caused had the Cubans launched the missiles on the U.S (Dipert 42).  Other popular historical examples where destruction was lessened by anticipatory actions include the 1981 bombing of Iraq by Israel to destroy a nuclear reactor and the War of the Spanish Succession (Dipert 42). Of course, it is often difficult to use these examples as "proof" that anticipatory action is justified in some situations because of the very nature of historical accounts and the way in which "the facts" are interpreted differently by various groups of people for political, religious, or other reasons.  
As has been shown, pre-emptive war, even in an age of terrorism, is not always wrong because the enemy's attack is imminent and known, the right to life of the enemy has been forfeited by their intention to steal that right from others, and pre-emptive war can minimize the total destruction of a war situation. The justification of pre-emptive war is difficult in that it often stretches the boundaries of traditional just war theory, and just cause is satisfied for many and various reasons. It has been shown here that pre-emptive war is not always wrong because the war has already begun when a nation knows of an imminent enemy attack. Pre-emptive war is also not wrong both because the right to human life must be protected and because the attacking nation has forfeited its right to life categorically, a forfeit that is so clearly seen in the case of terrorism. Finally, the potential of minimizing the total destruction of war by an anticipatory action further shows that even in an age of terrorism, pre-emptive war is not always wrong.
 This argument uses emotion as a way of knowing. People "feel" that self-defense is more innocent than aggression, which is an emotion that often carries negative connotations.
 These arguments rely on revelation as a way of knowing truth.
 One can see how Kant knows that there is value to human life apart from anything shown to him through revelation. He instead uses reason, largely through definition of terms, rhetoric, and deduction, and sometimes even an appeal to "common sense," to come to this understanding (and the entire understanding of retributive justice, for that matter).
 This uses aesthetics as a way of knowing. In his Poetics, Aristotle explains that "the arts" communicate universal concepts through particular means (Aristotle). In this case, the universal concepts of fear, grief, and even revenge were conveyed through the actual sight of the 9/11 attacks and the news coverage and pieces of are that were created in response.
 This information about the destructive abilities of nuclear weapons is known through science. Obviously, these weapons are a result of scientific work, but their effects have also been hypothesized and tested throughout history (sometimes at great costs), so the scientific method has been at work not only in their creation, but also in their testing.
 In this example, it was not "known" that Cuba was planning to launch these missiles on the U.S., so the criteria of a certain imminent attack was not met. So, if this "preventive" move from the U.S. was justified, then the condition that the total amount of destruction can be minimized by a pre-emptive/preventive strike is strong enough by itself in some situations to justify the pre-emptive/preventive actions.
 But, if enough people seem to agree that one of these historical examples shows justification for pre-emptive action, consensus gentium could be claimed as a way of knowing that this is true (Hillmer).
 These historical examples use science as a way of knowing because the scientific method can be seen in this argument on a macro level. A hypothesis was made that the amount of total destruction could be minimized by a pre-emptive attack in some situations. The hypothesis was then "tested" in these historical examples. After the results of the experiments were gathered, meaning in these cases that the total amount of damage was assessed and compared to the predicted damage of the imminent enemy attack, a conclusion was drawn. Just as scientific theories continued to be honed and revised by the scientific method and repeated experiments, a concept known as defeasibility (Dowe 193), this hypothesis continues to be tried by historical situations, some of which seem to prove and some disprove it.
Aristotle. Poetics. Trans. S. H. Butcher. Malcolm Heath, 1998. Web. 8 Dec. 2010.
"Declaration of Independence." 1776. Print.
Dipert, Randall R. "Preventive War and the Epistemological Dimension of the Morality of War." Journal of Military Ethics 5.1 (2006) : 32-54. Print.
Dowe, Phil. Galileo, Darwin, and Hawking. Grand Rapids, MI: Eerdmans Publishing Co., 2005. Print.
Hillmer, Paul. Ways of Knowing: How to think like an Honors Student. Concordia University. St. Paul, MN. 13 Sep. 2010. Lecture.
"Nuclear Explosions: Weapons, Improvised Nuclear Devices." Radiation Emergency Medical Management. U.S. Department of Health and Human Services, n.d. Web. 8 Dec. 2010.
"Pre-emptive war - Definition." wordiQ.com. n.p., n.d. Web. 8 Dec. 2010.
"terrorism." Collins Dictionary of Sociology. London: Collins, 2006. Credo Reference. Web. 08 December 2010.
Walzer, Michael. Just and Unjust Wars. New York: Basic Books, Inc., Publishers, 1977. Print.
White, James E., ed. Contemporary Moral Problems. 6th ed. Belmont, CA: Wadsworth Pub. Co., 2000. Print.
"wrong." The Macquarie Dictionary. South Yarra: The Macquarie Library Pty Ltd., 2005. Credo Reference. Web. 08 December 2010.