While, on balance, it is unlikely that the TPNW could contribute to a customary international law ban on nuclear weapons, this is not to say that NATO’s concerns in this regard are misplaced.
The 2017 NAC statement asserted that Allies ‘would not accept any argument that this treaty reflects or in any way contributes to the development of customary international law’. The 2020 NAC statement repeated this language. It is unusual for NAC statements to contain detailed language of this sort on international law issues. This is a sign of how heavily the concern that the TPNW might somehow contribute to a customary international law ban on nuclear weapons has weighed on the debates among Allies. On balance, it is unlikely that the TPNW could have this effect, given the current state of international law. However, this is not to say that NATO’s concerns are misplaced.
Underlying the concerns set forth in the two statements was a sense in some quarters that the TPNW was being used as the latest front in a decades-long legal strategy to create an overarching norm of international law against nuclear weapons. There was also a concern that the TPNW itself was not sufficiently nuanced in its portrayal of the state of international law on nuclear weapons. The TPNW’s preamble states that ‘any [emphasis added] use of nuclear weapons would be contrary to the rules of international law applicable in armed conflict, in particular the principles and rules of international humanitarian law’. While the preamble refers to ‘any use’, the International Court of Justice’s 1996 advisory opinion on the Legality of the Threat or Use of Nuclear Weapons did not go quite so far. Rather, the Court held, in a divided vote: […] the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict, and in particular the principles and rules of humanitarian law;
While the entire advisory opinion was a controversial one (and, like all advisory opinions, non-binding), the omission of the in extremis caveat from the TPNW’s preamble did not go unnoticed. These concerns might help explain why the two NAC statements address the legal consequences (or lack thereof) of the TPNW. As the 2017 statement put it, ‘[…] there will be no change in the legal obligations on our countries with respect to nuclear weapons. Thus, we would not accept any argument that this treaty reflects or in any way contributes to the development of customary international law.’
While it is a general principle of international law that treaties do not create obligations for third states, it is also an accepted principle that a rule set forth in a treaty could, under certain conditions, become binding on a third state as a customary rule, as long as the dual requirements of state practice and opinio juris (the recognition of a norm as legally binding) are present in the first place. However, this is far from an automatic process. Two distinct concepts are relevant here: the concept of so-called ‘specially affected states’, and that of ‘persistent objectors’. These concepts are distinct and must be analysed separately. As the ICJ has explained, a lack of consent from specially affected states may have the effect of preventing the required general state practice from emerging, preventing the rule from coming into being in the first place. There is a strong argument that states with nuclear weapons and those in a nuclear alliance would be specially affected by a proposed ban on nuclear weapons. Even if a rule is indeed created, states that have objected to a certain degree to its emergence – so-called persistent objectors – will not be bound by it. While some elements can be extrapolated, there are no specific requirements of international law for how – including how often and in what format – persistent objectors need to make their position known. A pragmatic and contextual approach would be needed to evaluate a claim that a state is a persistent objector.
In the current circumstances, the NAC statements appear to be an effective strategy, both for preventing the emergence of a rule of customary international law in the first place and for establishing NATO Allies as persistent objectors to such a rule. We might expect NATO Allies to consider measures to maintain the effectiveness of this strategy over time. This could include reaffirming the NAC statements from time to time, potentially on the occasion of key events such as the first meeting of the TPNW states parties under the treaty.
One practical recommendation in this area would be for NATO to convene discussions between legal experts from Allies on a regular basis in order to explore potential steps in light of the evolution (for better or worse) of the international and regional security landscape.
