The new first line of defense.(Scan)

National security has always depended on our ability to see our adversaries' defensive arrangements clearly, and their ability to see ours. As long as each side can see what the other is doing, neither need fear a surprise attack.

Before the advent of weapons of mass destruction, our first line of defense was a territorial border. But such weapons are so easy to hide and move around that this line of defense now runs deep into the bureaucracies of our adversaries. In rogue states, the bureaucracy itself is a shadowy world of criminal intrigues and terrorist plots. Without transparency in their WMD programs, the danger of sudden catastrophe is always present.

Jurists fiercely debate the legality of President Bush's doctrine of "preempting" such threats before they have fully formed. Article 2 of the United Nations Charter prohibits the use of force, so the question is the scope of Article 51, the Charter's "self-defense" provision. Article 51 protects "the inherent right of self-defense," but only "if an armed attack occurs." Whatever this may mean, the real debate has revolved around whether a threat is imminent enough to justify a preemptive strike.

But the word "imminent" doesn't appear once in the Charter. It does not even appear in the nineteenth-century exchange of diplomatic letters between Britain and America related to the Caroline dispute--the classic precedent for the "imminent threat" rule. The proposition that only an imminent attack can justify a preemptive strike is an academic invention that has gained currency only through repetition. The Caroline letters, in fact, stand for a very different proposition: Self-defense is justified when it is necessary.

But the Charter almost certainly supercedes the Caroline precedent. Many scholars, especially abroad, thus argue that international law prohibits preemption of even the most "imminent" attack. According to this view, even if your neighbor has declared war and is piling up forces on your border, your only hope is the Security Council. This absurd interpretation has an unfortunate virtue: It corresponds exactly to what the Charter actually says.

Luckily, international law provides rules for interpreting treaty provisions that are "manifestly absurd or unreasonable." The first place to look is the notes from a given treaty's preparatory work--in this case, the records of the negotiations culminating in the San Francisco Conference of 1945, at which the U.N. was ...