Nuclear Weapons and International Law: The Impact of the Treaty on the Prohibition of Nuclear Weapons

Written by

With Honduras being the 50th state to ratify the Treaty on the Prohibition of Nuclear Weapons (TPNW) on the 24th of October 2020, the 90 day countdown has begun for the treaty to come into force. Honduras also happened to ratify the treaty on UN Day, and not just any UN day, but the 75th anniversary of the United Nations. That means the TPNW will come into force on 22 January 2021. Maybe it will be an omen of 2021 being less of a bonfire than 2020.

In the face of the perceived general trend away from multilateralism, this multilateral treaty prohibits the development, testing, production, stockpiling, stationing, transfer, use and threat of use of nuclear weapons, as well as assistance and encouragement for any the prohibited activities for State Parties. It is a major step in the banning of nuclear weapons – by actually banning them. The Treaty on the Non-Proliferation of Nuclear Weapons (NPT) was supposed to lead to eventual disarmament, but after 50 years there is growing discontent with the compliance of states with the treaty obligations, and the ability, under the treaty, to induce this compliance.

In this post I offer some background to the TPNW, consider the relationship between the NPT and TPNW, and reflect on the impact of the TPNW on international law and the legal status of the obligations contained therein.


In 2017, the UN General Assembly decided to convene a ‘conference to negotiate a legally binding instrument to prohibit nuclear weapons’. The conference took place in New York, and after much negotiation the treaty text was adopted on 7 July 2017 (with 122 states voting in favour) and opened for signature on 20 September 2017.

Along the way, the International Campaign to Abolish Nuclear Weapons (ICAN) was awarded the Nobel Peace Prize ‘for its work to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons and for its ground-breaking efforts to achieve a treaty-based prohibition on such weapons’. The impact of non-state actors at all stages of its negotiation and the campaign for ratification has been very significant and unusual. Useful comparators in that regard would be the Rome Statute of the ICC and the Landmines Convention, both of which also saw non-state actors heavily involved.

The Relationship Between the NPT and TPNW

There is of course another anniversary this year: 2020 marked the 50th year since the entry into force of the NPT in 1970. There has been some discussion about whether the NPT and TPNW can ever co-exist together. Two different regimes governing nuclear weapons appear on the face of it to have been created with potentially conflicting obligations. France, the United Kingdom, and the United States (noting their general positions on nuclear weapons) claimed the TPNW would ‘undermin[e] the existing international security architecture which contributes to the maintenance of international peace and security’.

Arguably the NPT’s greatest success, despite the dissatisfaction on the part of those who argue it never went far enough, has been that it successfully froze the number of nuclear states. It is thought that there are only 9 nuclear armed states, and that number has been relatively steady for the last 20 years. At a time when it seems that ‘the international security environment has become increasingly unsettled’, freezing the number of nuclear states should perhaps be considered an achievement. However, given that there are ‘growing concerns not just that new nuclear arms reduction agreements are very unlikely, but that existing agreements, including the Intermediate-Range Nuclear Forces (INF) Treaty, are unraveling and that this could lead to renewed nuclear arms competitions’, it is encouraging that things are going forwards not backwards on the prohibition of nuclear weapons.

Rather than undermining the NPT, the TPNW could potentially be interpreted as the fulfilment of Articles 6 and 7 of the treaty. Article 6 obliges state parties to ‘pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control.’ Kai Ambos and Matthias Lippold recently wrote on this blog about the potential for this obligation to be customary international law. The TPNW is that treaty, even if the hope underlying Article 6 might be the eventual replacement of the NPT. In terms of the relationship between the two treaties, Article 7 explicitly states that ‘Nothing in this Treaty affects the right of any group of States to conclude regional treaties in order to assure the total absence of nuclear weapons in their respective territories.’ Article 7 would seem to reject the idea proffered by some states that the NPT will be undermined by the presence of the TPNW, instead making room for the suggestion that it should be welcomed and that treaty obligations under the TPNW would take priority over any under the NPT.

Many eyes will be on the postponed Review Conference of the Parties to the NPT that was meant to be held this year. Due to the pandemic, State Parties have agreed to postpone the Tenth NPT Review Conference to no later than August 2021, by which point the TPNW will have entered into force. The delay might be fortuitous given the postponed date allows for more concrete negotiations and discussions to take place.

Nuclear Weapons and International Law as a result of the TPNW.

In the following, I aim to provide a systematic clarification of the relevant international law considerations in an accessible manner to offset some of the recent overexcited pronouncements proclaiming nuclear weapons to be illegal full stop.


Before looking at the dynamics of treaty ratification and the customary international law status of any obligations, I first address what the obligations are. Focus has, in the past, been on ‘the use or threat of use’ of nuclear weapons and there are a number of different potential activities surrounding nuclear weapons that feature in international law.

First, as referred to above, there is the obligation to pursue negotiations in good faith on nuclear disarmament that is treaty-based and potentially also exists in customary international law. That does not mean that nuclear disarmament is an obligation but that states are obliged to pursue negotiations in good faith as per Article 6 of the NPT.

As regards a prohibition to threaten use or use nuclear weapons, it is rather more complicated. In its 1996 Advisory Opinion on the Threat or Use of Nuclear Weapons, the ICJ determined unanimously that there was no ‘specific authorisation of the threat or use of nuclear weapons’ and determined 11 to 3 that neither is there in ‘customary nor conventional international law any comprehensive and universal prohibition of the threat or use of nuclear weapons as such’ (para 105). In many ways the TPNW leaves this finding unchanged. There is now a comprehensive provision for some states in a treaty (the TPNW). However,  this is not a universal prohibition unless or until all states sign the TPNW or the obligation attains customary international law status.

In this same Opinion, the ICJ found threat or use of force by means of nuclear weapons would be contrary to Article 2(4) of the UN Charter. The prohibition of the use of force is customary international law and generally considered to be a jus cogens rule. But that obligation covers the use of force by any means, including nuclear weapons.

The Court found unanimously that any threat or use of nuclear weapons must be compatible with the international law applicable in armed conflict, mentioning specifically international humanitarian law. As a result, 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’ (para 105). As we might surmise from experience, nuclear weapons are unable to distinguish between combatant and civilian. What the Court could not definitely conclude however (and the bench was divided on this 7-7 with President Bedjaoui having the casting vote) was ‘in view of the current state of international law…whether the threat or use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defence, in which the very survival of a State would be at stake’ (para 105).

The TPNW has changed the position in respect of this conclusion. Fifty states at least, even in an extreme situation where their survival is at stake, cannot threaten to use or use nuclear weapons under Article 1. The TPNW accepts no reservations to its obligations (Article 16) so states cannot create their own exceptions for extreme circumstances. With the TPNW entering into force, there is now no doubt as to there being an explicit prohibition on the use or threat of use of nuclear weapons, without exception, under international law – but one, of course, that only applies to State Parties to the treaty.

For those states that are party to the TPNW, the treaty prohibits the development, testing, production, stockpiling, stationing, transfer, use and threat of use of nuclear weapons, as well as assistance, encouragement or inducement of anyone to engage in any of the prohibited activities (Article 1). Article 1 is a comprehensive prohibition of many different activities that relate to nuclear weapons. Before the TPNW the focus has primarily been on the ‘use or threat of use’ of these weapons.

Finally, there are many other obligations that the TPNW now includes. I will discuss some of these in the context of the UK’s domestic political situation and where to next for nuclear disarmament below.

Treaty Ratification – Dynamics

An obvious point, but one which bears restating is that the TPNW is only binding for State Parties. There are now 50 ratifications, but with 122 states having voted in favour in 2017 of adopting the treaty text, and 84 signatories, the areas of the world where nuclear weapons are banned will likely increase. This is something that the US appears rather concerned about. Recently, the US put pressure on states that have already ratified to withdraw their ratifications, saying that ‘although we recognize your sovereign right to ratify or accede to the Treaty on the Prohibition of Nuclear Weapons (TPNW), we believe that you have made a strategic error and should withdraw your instrument of ratification or accession’. So far it appears that no state is bowing to this pressure, although this is difficult to measure given that it may make some states think twice about ratifying the instrument.

The first instrument of ratification was deposited on 16 January 2018 by Mexico, but it took nearly three years for the TPNW to reach 50 ratifications after the treaty was opened for signature on 20 September 2017. There are 84 states in total that have become signatories to the TPNW, so we might expect another 34 ratifications, although none have been forthcoming since Honduras. Of course, the pandemic has occupied the attention of states so we cannot read too much into this plateau of ratifications as a lack of support for TPNW. We may find that with the entry into force in January, we see another flurry of ratifications.

In contrast to this, the dynamics surrounding the entry into force of the NPT was rather different. This treaty contains a ratification process found in Article IX, which required the ratification by the ‘designated Depositaries’ States (UK, (the then) Soviet Union, and US) plus forty other states and came into force once these ratifications were deposited. Opened for signature on 1 July 1968, the NPT more than received the requisite number of signatories comfortably inside the first two years (at my count 59). But importantly, as we will come to see in relation to customary international law below, the NPT was ratified by many nuclear weapon states in the first two years (UK 1968; US 1970; (now) Russian Federation 1970) and has since received ratification by France, China and (although they withdrew) North Korea. Also worth noting in this regard: South Africa got rid of its nuclear weapons just before acceding to the NPT in 1991, as did Belarus in 1993, Kazakhstan in 1994, and Ukraine in 1994.

Returning to the TPNW, full lessons from its ratifications and uptake remain to be seen. The only metrics we can currently compare is the level of state ratification between being opened for signature and entry into force. It would be too premature to take much away from this, aside from noting that the NPT saw quicker uptake and significantly that there was support for the treaty among more nuclear weapon states.

Customary International Law

This exercise is more than just an interesting aside. It speaks to an important issue about whether or not the prohibition of the use or threat of use of nuclear weapons has attained or might attain customary international law status.

It is extremely unlikely that any or all of the obligations found in the TPNW will in the near future become customary international law at a global level. There are a number of reasons for this. To assess these reasons, focus must be paid to both the practice of states that sign the treaty and to those who do not sign the treaty (and who they are).

First, one in four states’ ratifying the treaty is never going to count as practice that can create custom. Practice is generally required to be ‘widespread and representative’ (para 73, North Sea Continental Shelf Cases) if this were to be the case. Patently fifty states ratifying the treaty meets neither of these criteria.

Looking to the months and years ahead, attention must also be paid as to why states are signing and ratifying the treaty. Potentially, the en masse signing of the TPNW may be conceived of as evidence of state practice but there must also be evidence that states are joining the treaty because they think themselves obliged to do so in order to satisfy the opinio juris requirement. Indeed, here is where we run into Baxter’s paradox: we would have less idea what state practice and opinio juris is outwith the TPNW and this opportunity decreases the more states sign the treaty. The obligation not to use or threaten to use nuclear weapons does not appear to have been emerging as custom prior to the TPNW – indeed, the very idea of deterrence is most likely based in a threat – thus, there is insufficient evidence to suggest that the TPNW codified custom.

Finally, even if all 122 states that voted to approve the treaty text in the UN General Assembly, accede to the treaty after its entry into force, attention must be paid to the state practice of those states that have nuclear weapons as well as the narrative that might be built around their practice. For in order to have custom emerge ‘without the passage of any considerable period of time’ not only must there be ‘widespread and representative’ practice, but that practice must include those states ‘whose interests were specially affected’ (para 73, North Sea Continental Shelf Cases). An obvious reading of states whose interests are specially affected are those states with nuclear weapons. They all appear unlikely to sign the TPNW currently, and their practice is significant for the formation of custom.

A potential reading of the concept of ‘specially affected’ states is one which might include those states in whose territories and against whose populations nuclear weapons have been used and / or tested. However, perhaps these states may be considered specially affected only with regard to those customary rules that concern past usages or testing of nuclear weapons. Whether or not they can also be considered specially affected states with regard to rules addressing potential usage, threats to use, or testing, is another matter. 

In the unlikely event (unlikely at least in the near future for all the reasons outlined above) that every single state in the world apart from nuclear weapon states sign the treaty and make explicit statements so as to satisfy the opinio juris requirement, and custom does emerge after the passage of a considerable period of time, nuclear weapons states may still not be bound by the prohibition if they persistently object. In short, in the current view of things, a universal prohibition is remote.

Where next for nuclear disarmament?

As we have seen, how the practice of states is framed is vital and the attitudes of a few states can be overly determinative. The intransigent attitudes of nuclear weapons states seem unlikely to change. Yet if they do, a lot could change overnight.

The domestic political situation in many of the nuclear weapons states might shift nuclear policy and the seemingly entrenched positions of individual states. The most vulnerable state here in many ways is perhaps the UK.

The UK is one of the major nuclear armed states, but its nuclear weapons and submarines are located in Scotland. As established in earlier UK Parliamentary inquiries in relation to the 2014 Scottish independence referendum, there appear to be very few or no suitable alternatives to Coulport – the Royal Navy Armaments Depot, where the nuclear warheads are stored. An rUK (‘rest of the UK’) government may find it tricky to relocate its nuclear warheads even if it could relocate the submarine base (currently stationed at Faslane, HMNB Clyde). In some polling there appears to be a majority in favour of nuclear disarmament in Scotland, and now most polls show some kind of small majority in favour of independence. There are grumblings about holding another referendum sooner rather than later, and it seems the current First Minister’s preference is to have one early in the life of the next Parliament. Given the two pro-independence parties support the TPNW (see here and here), if Scotland were to become independent it seems likely it could sign the TPNW. There has been a telling uptick in opinions and positioning on Trident coming from the British establishment.

If an independent Scotland were to sign the TPNW, Article 1 provides that states should not ‘allow any stationing, installation or deployment of any nuclear weapons or other nuclear explosive devices in its territory or at any place under its jurisdiction or control’. Under Article 4 ‘each State Party that has any nuclear weapons or other nuclear explosive devices in its territory or in any place under its jurisdiction or control that are owned, possessed or controlled by another State shall ensure the prompt removal of such weapons, as soon as possible’. Both provisions would create problems for an independent Scottish Government approving some kind of a lease of Faslane and Coulport to the rUK government. But let us not get ahead of ourselves, the Scottish Government – or a pro-independence majority – would need to win in the 2021 elections first.

Looking from the domestic to the international political situation, there are the actions of other states which might help break through the ‘deterrence’ discourse deadlock. There is a notable obligation in the TPNW in Article 12 in relation to universality. That provision obliges states to ‘encourage States not party to this Treaty to sign, ratify, accept, approve or accede to the Treaty, with the goal of universal adherence of all States to the Treaty’. It thus becomes the obligation of fifty states to encourage others to change their attitude towards nuclear weapons. Perhaps a good number of states acceding to this treaty will become a catalyst for those once intransigent attitudes.

All this is for another day perhaps. The immediate ‘what next’ is the entry into force of the TPNW on 22 January 2021. A day that many of those who are pro-disarmament and who have campaigned for decades will celebrate regardless.

Print Friendly, PDF & Email

Leave a Comment

Comments for this post are closed


Nicolas Boeglin says

December 2, 2020

Dear Professor Lythgoe

Many thanks for this extremely interesting note and the very valuable information provided.

On July 7th, 2017 when Ambassador Elayne Whyte (Costa Rica) as Chair asked if the text could be adopted by consensus, the delegation of The Netherlands asked for a vote: a few minutes after, the result was 122 in favour, one against (The Netherlands), and one abstention (Singapur).

In your view, why exactly The Netherlands asked for a vote? I do not have in mind any vote where The Netherlands appear so isolated at the United Nations, but maybe I´m missing other similar situations.

Politically speaking, what could be the idea to show that a State is alone (completely alone) voting against such a text ? Not just at a global level, but also at a regional level.

It must be noted that among EU members, Austria, Cyprus, Ireland, Malta, and Sweden voted in favour, The Netherlands against and that all the others EU members decided not to register a vote (“No Show”), including France and UK.

Sincerely yours

Nicolas Boeglin

Jane Hofbauer says

December 3, 2020

Thank you for this interesting post! I was wondering - do you see, in strictly legal terms, any incompatibility between the TPNW and the NPT? It seems you and others argue that this is not the case and that the concerns are rather political..

Also, for a further detailed analysis of NPT and TPNW, I can highly recommend the following contribution, which touches upon several of the ideas addressed in this article:

'In search of the elusive conflict: The (in-)compatibility of the Treaties on the Non-Proliferation and Prohibition of Nuclear Weapons' (by Michael J. Moffatt)