The ratification on 25 October of the Treaty on the Prohibition of Nuclear Weapons (TPNW) by Honduras, the 50th nation to sign, marked the beginning of the final chapter of the legitimacy of nuclear weapons.

Even without the nuclear-armed states and their allies, the TPNW will now automatically enter into force on 22 January 2021, and immediately set a new benchmark against which all other nuclear disarmament measures will be judged.

The treaty’s activation will begin to shift the international legal norm and generate a stigma around these cruellest of indiscriminate weapons. This will have ramifications for defence policy, military doctrine, weapons manufacturing, banks and super funds, as was the case when cluster munitions, chemical and biological weapons, and landmines were outlawed.

The TPNW couldn’t be more timely. Numerous organisations, including the International Red Cross and Red Crescent Movement, the 2017 Nobel-prize winner ICAN, and the Bulletin of the Atomic Scientists have judged that the risk of nuclear conflict is higher now than it has been for decades. This is because nuclear-armed states are expanding their arsenals by, for example, including smaller, tactical atomic weapons; modernising their nuclear-weapon delivery systems to include underwater drones and nuclear-capable hypersonic missiles; and abandoning longstanding arms-control agreements. Some, too, are vulnerable to cyberattacks.

Last month, 56 former world leaders were in furious agreement with humanitarians, civil society and the science. Their unequivocal statement in support of the TPNW asserted that without a doubt ‘a new nuclear arms race is underway’.

Here was no pollyannaish crowd of flower-holding peaceniks. The gathering included two former secretary-generals of NATO and one of the United Nations, prime ministers, foreign ministers and defence ministers from 20 NATO member countries plus South Korea and Japan, all urging their governments to join the treaty. Having such diverse backing is one of the TPNW’s greatest strengths and why it will eventually upend the status quo.

Since the treaty’s adoption in 2017, nuclear-armed states and allied nations have denounced it as weak and a distraction that will undermine the existing legal architecture, the cornerstone of which is the Nuclear Non-Proliferation Treaty (NPT) drafted in 1968.

While the NPT was a monumental accomplishment, its implementation stagnated as nuclear-armed states came to believe that they were entitled to maintain their own nuclear weapons in perpetuity. Its integrity eroded as it repeatedly failed to fulfil its aspiration to ‘facilitate the cessation of manufacture of nuclear weapons, the liquidation of all existing stockpiles, and the elimination from national arsenals of nuclear weapons and the means of their delivery’.

Disappointingly and fatally, it made no substantial progress on the key obligation 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’.

While the NPT will remain a key pillar of the legal architecture with a leading role to play, the TPNW is designed to complement it and remedy its critical shortcomings while reinforcing the same norms and institutions championed by the NPT.

Some nuclear-armed states and their allies continue to argue that these weapons are a necessary component of their defence posture and that they keep us safe. Former NATO leaders disagree and argue strongly that these weapons unleash obscene humanitarian consequences. As long as there are nuclear weapons there is a risk that they will be used, intentionally, by accident or by miscalculation, and no adequate humanitarian response can be mounted.

The argument ignores the shocking and painful deaths and injuries inflicted on hundreds of thousands of Japanese by two relatively small bombs in 1945. Today in Hiroshima and Nagasaki, Red Cross hospitals continue to treat the survivors and research is being conducted to determine whether the illnesses being experienced by their descendants two generations later can be explained by mutations in their DNA caused by radiation.

Nuclear-aligned countries, such as Australia, were reticent to engage with the TPNW treaty-making process, reasoning that it may be inconsistent with their legal obligations under their defence arrangements.

But a recently published legal analysis of whether joining the TPNW would undermine the ANZUS security treaty found that it creates no legal impediment. The ANZUS treaty makes no reference to nuclear matters, and even if subsequent practice and statements have effectively redrawn its terms, there is still no legal barrier to entering negotiations to vary it. Similarly, the former world leaders said their governments ‘could remain in alliances with nuclear armed states, as nothing in the [NATO] treaty or our defence pacts precludes that’.

In the commercial sector, the onset of the TPNW was being felt even before Honduras’s ratification. The flow of investment funds away from nuclear weapons manufacturers has been steadily increasing. Sixteen Japanese banks, two of the top five major global pension funds, the Norwegian Government Pension Fund, KBC Bank Ireland, Deutsche Bank and others have now divested or in the process of divesting.

Significant numbers of banks, super funds and pension funds have included in their environmental social governance frameworks commitments not to fund controversial weapons. Nuclear weapons should now, belatedly, be assigned to this category and excluded from their portfolios. Manufacturers of nuclear weapons and their banks and shareholders must re-examine their policies, practices and investment screening criteria to preserve their reputations, avoid regulatory risks and stranded assets, and demonstrate to shareholders that they are behaving in accordance with international standards and best practice.

As the International Red Cross and Red Crescent Movement welcomes the January 2021 commencement of the nuclear weapons ban regime in the midst of a global pandemic, we remind all governments that Covid-19 has taught us that even low-probability events can and do occur, to devastating effect.

Just like the Covid-19 response, we will eliminate this threat only with sensible, ethical, global action. There will never be a vaccine for the bodily effects of nuclear weapons or their impact on our fragile environment. Prevention is the only course. We have worked towards this new era for 75 years, motivated by the interests of humanity and the principles of international humanitarian law. We hope that all states will join the 50 early adopters and support this sensible, ethical treaty sooner rather than later.