The aims of the Kellogg-Briand Pact were adopted in the Charter of the United Nations in 1945. Under the charter, the use or threat of force as an instrument of national policy was condemned, but nations were permitted to use force in individual or collective self-defense against an aggressor. The General Assembly of the United Nations has further defined aggression as armed force by a state against the sovereignty, territorial integrity, or political independence of another state, regardless of the reasons for the use of force. The Security Council is empowered to review the use of force, and therefore, to determine whether the relevant circumstances justify branding one nation as the aggressor and in violation of charter obligations. Under the modern view, a just war is one waged consistent with the Kellogg-Briand Pact and the Charter of the United Nations. [From "Just War," Eds. Jeffrey Lehman and Shirelle Phelps, 2005.]There's no need for a negative going up against a "just war theory" to try and defeat all of the theory (though it's possible). Instead, the negative can argue that a preemptive strike by the United States would violate the Kellogg-Briand pact and the UN Charter.
What about an aff response? There are reasons to question the UN's legitimacy.
No permanent and impartial international body has been created to administer the rules of war. Although the United Nations has acted with multinational support in the Korean and Gulf Wars, and the International Court of Justice has adjudicated claims against democratic and totalitarian regimes alike, neither body exercises sovereignty over individual member states in any meaningful sense, and powerful countries generally wield more influence over these bodies than do weaker countries. [From "Rules of War," Eds. Jeffrey Lehman and Shirelle Phelps, 2005.]Thus, to any negative running a UN case, the aff has a potential block. Not only is the UN practically ineffective, it's principally unprepared to adjudicate conflicts. When it comes to a nuclear threat, the United States can't wait for the UN to act.