Any reasonably imaginable use of nuclear weapons would breach the long-standing laws applicable in armed conflict (such as proportionality, impact on non-combatants, i.e. civilians, and long term environmental damage). It is also recognised that if the use of a weapon is illegal then the threat to use it is also illegal. States which possess nuclear weapons are de facto threatening to use them and so their possession of nuclear weapons, to say nothing of active deployment, is in itself illegal.
The Treaty on the Prohibition of Nuclear Weapons (TPNW) builds on this legal basis to specifically prohibit nuclear weapons as a class, similar to the bans on chemical weapons, landmines etc. While nuclear-armed states which are not signatories to the TPNW are not legally bound by its provisions they are bound by the laws of armed conflict and by humanitarian law. Their doctrine of “deterrence” is based on a position of permanent threat and is therefore illegal.
As well as the general threat involved in “deterrence”, over the years the nuclear-armed states have made quite specific threats to use nuclear weapons. In the UK’s case there was the threat from Defence Minister Geoff Hoon in 2003 to respond with nuclear weapons to any chemical attack on UK troops in the Iraq war. The US also threatened their use in the Korean War. So Putin’s horrific threats are not unique. When the US President condemned what he called Russia’s “irresponsible” threats the implication is that the threat carried by the US arsenal is valid and responsible. As far as nuclear weapons are concerned there are no responsible threats. They are always in the wrong hands. Only wrong hands can hold them.
In the nuclear era e have faced a number of crises when the world was close to nuclear catastrophe, saved only luck or by leaders drawing back from the brink at the last minute. This is why the TPNW aims for the total elimination of nuclear weapons.