Tuesday, the U.S. Department of Defense presented its latest Nuclear Posture Review Report. I haven’t had a chance to read the entire document yet, but media reports have focused on a new policy declaration that is of great interest to states and scholars alike.

The statement garnering the greatest attention is included in the “Executive Summary” of the NPR (p. viii):

The United States will continue to reduce the role of nuclear weapons in deterring non-nuclear attacks.

To that end, the United States is now prepared to strengthen its long-standing “negative security assurance” by declaring that the United States will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear non-proliferation obligations.

Essentially, the U.S. is reversing longstanding nuclear policy by promising merely to employ “devastating conventional military response” against threats it previously used nuclear weapons to deter: potential chemical or biological weapons (CBW) attacks against the U.S. or its allies. The document makes this explicit, noting that even though the U.S. had abandoned its own CBW programs, it “reserved the right to employ nuclear weapons to deter CBW attack on the United States and its allies and partners.”

Among scholars, this development is interesting because it potentially contributes to strengthening a norm (or perhaps tradition or taboo) of non-use of nuclear weapons. As McGill’s T.V. Paul argues in the book that I just linked, the U.S. refusal to preclude the threat of nuclear retaliation against states using CBW had long weakened the tradition — to the dismay of non-nuclear weapons states everywhere. In fact, during the last decade other nuclear-armed states had followed the U.S. lead and weakened prior non-use pledges in the face of CBW threats in the post-9/11 era.

By excluding the threat of nuclear retaliation against CBW attack, the U.S. is now potentially strengthening the tradition (or norm or taboo) and could serve as a role model for other states that emulated its more threatening previous posture.

Non-nuclear weapons states are likely to be pleased by the new U.S. declaratory strategy since many of them have been arguing since the 1960s for these kinds of “negative security assurances.” It was a point of contention even in the original NPT debates.

Before anyone gets too excited about the U.S. announcement, it should be noted that Iran and North Korea are excluded from the U.S. promise. These states, now apparently called “outliers” rather than “rogue states” by the U.S., have now been explicitly warned that they could still suffer a nuclear blow if they used CBW against the U.S. or its allies.

Indeed, even as the NPR reduced the number of nuclear threats the U.S. is making, Defense Secretary Robert Gates also arguably increased them. By isolating and highlighting the “outliers,” the U.S. is essentially trying to leverage a nuclear threat for counterproliferation purposes:

“If there is a message for Iran and North Korea here, it is that if you’re going to play by the rules, if you’re going to join the international community, then we will undertake certain obligations to you. But if you’re not going to play by the rules, if you’re going to be a proliferator, then all options are on the table in terms of how we deal with you,” said the secretary of defense.

Still, Gates called the use of nuclear weapons a “last resort.”

This statement amounts to a renewal of the Bush Doctrine, linking the potential first use of military force — in this case nuclear weapons — to counterproliferation aims. As Phil McCauley and I recently warned, the fears about biological weapons proliferation are sufficiently strong that they render the current taboo against their use illogical by classic arms control standards as they increase the risk of war.

The U.S. needs to couple the new policy with active efforts to strengthen the chemical and biological arms control and disarmament regimes as well. It was the U.S. after all, that blocked the negotiated verification protocol to the Biological Weapons convention just months after the 9/11 attacks.