1. The horrifying, long-term, indiscriminately destructive power of nuclear weapons.
2. The new wave of martyrism among members of certain states possessing nuclear weapons, thus negating the deterrent power of Mutually Assured Destruction.
3. The global and transnational nature of the threat, where the consequences are not defined by state borders.
There may even be a better reason. Following Daniel Zupan's analysis in War, Morality And Autonomy: An Investigation in Just War Theory, I argue that the moral nature of nuclear weapons* justifies preemptive action by the United States** to prevent their acquisition by states posing a military threat.
Zupan shows that nuclear weapons, by their very nature, destroy the moral fabric of war, a fabric carefully sewn up in the years following the horrors of the Second World War. He writes, "The intention of the weapon is to undermine the war conventions and obliterate the distinction between combatant and noncombatant." In effect, a nuclear weapon is a war crime waiting to happen. Zupan argues that this would allow us to strike against a laboratory where civilian scientists are working on nuclear weapons:
We can target the lab, and only the lab, and we can do so because there really is no other way to combat its product, and its workers really are assimilated into the ranks of combatants. We cannot wait to engage it on the field of battle because it is never intended to get to the field of battle. Since the weapon essentially goes from the lab to its target--granted, there are a few stops in between, but none of the stops are the battlefield--it is as if the scientists employ the weapon and in this sense, they are combatants.Thus, the United States, if faced with a military threat by a regime attempting to develop The Bomb, would have the rules of war on its side in using force to prevent that development. Although the resolution does not specify which sort of military action is justified, I think it's safe to say that the same reasoning would allow, say, a Cuban blockade or an Osirak scenario.
There is a meaningful sense in which we can say that, if we do not stop the production of the weapon, we will not get a chance to defeat it on the battlefield, once it gets into the hands of combatants. There will be no intervening battlefield to engage it combatant-on-combatant. In a very real sense it is reasonable to say that the weapon goes from the lab to our backyard. In this sense, the scientists employ the weapon and are, therefore, directly engaged in harming us. We are justified in defending ourselves against this type of threat.
Zupan's theory of just war, I should note, is based on Kantian moral scheme with humanity's value at the center. I might post a brief overview of his theory if enough debaters are interested.
*Other weapons of mass destruction might meet the criteria described here, which should not affect this analysis of the resolution. For all we know, the same principles could apply to, say, a nation manufacturing an anthrax weapon.
**I wouldn't shy away from noting that any nation threatened by a regime possessing nuclear weapons would be justified in a preemptive response. It might be an imprudent course of action for pragmatic reasons, though, even if justifiable. Declaring something "just" is not the same as saying it is obligatory.