Critics of nuclear weapons have long pointed to two sets of risks. First, deterrence stability depends on all fail-safe mechanisms working every single time in every bomb-possessing country. That is an impossibly high bar for nuclear peace to hold indefinitely. Second, it also requires that rational decision-makers be in office in all the world’s nine nuclear-armed states.

Since the dawn of the nuclear age in 1945, perceptions and strategic positions amongst States regarding nuclear weapons have always differed. The 2017 Treaty on the Prohibition of Nuclear Weapons (TPNW) has thrust these existing divisions to centre stage. The treaty is the result of an increasing emphasis placed by non-nuclear weapon States on the humanitarian consequences of nuclear weapons and the persisting risks posed to all humanity by these weapons.

I’ve spent my life studying the risks posed by nuclear weapons. I’ve always worried that one day an unhinged leader would emerge in a nuclear-armed country, with the authority to launch a nuclear attack. To me, the assumption that nuclear-armed leaders will act rationally has always seemed fundamentally flawed and dangerous. Robert McNamara, former US secretary of defence, highlighted this problem when he warned “the indefinite combination of human fallibility and nuclear weapons will destroy nations”. Today, I worry about this more than ever and wish more people would wake up to the problem and push for change.

Critics and proponents of the Treaty on the Prohibition of Nuclear Weapons both pay too little attention to its provisions on legal accountability for past and possible future detonations of these weapons. Among other things, the treaty calls on each State Party to “provide technical, material and financial assistance to States Parties affected by nuclear-weapon use or testing” and to “provide assistance for the victims of the use or testing of nuclear weapons or other nuclear explosive devices” [article 7]. Such victims could be non-belligerent nations who had no say or benefit in the decisions of nuclear-possessor states.

The Treaty on the Prohibition of Nuclear Weapons is a gamechanger. It puts all nuclear-armed states in breach of international law, along with any state that includes these weapons in its security doctrines. It coalesces regional nuclear weapon free zones, builds upon other nuclear arms control agreements and in ten simple pages condemns any activity that condones nuclear weapons.

The Treaty on the Prohibition of Nuclear Weapons (TPNW or the Ban Treaty) exposes the contradictory engagements of umbrella states. Many of them are strong supporters of nuclear disarmament while subscribing to alliance policies that underline the importance of nuclear weapons and sustain nuclear modernisation.