Not wrong. There is no law against the US, UK, France, Israel, or India having nuclear weapons.
There is no law against having any weapon, like nuclear weapons.
But there are international laws ratified by the US, against using them on people.
They fall under the International Humanitarian Laws the US agreed to in 1980.
The following discussion is why flame throwers are now considered illegal, as well as napalm and white phosphorus.
But they also fall under the chemical weapons laws.
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IHL Treaty law on incendiary weapons
The only international humanitarian law (IHL) treaty norms specifically regulating incendiary weapons are found in the 1980
Protocol III on Prohibitions or Restrictions on the Use of Incendiary Weapons to the Convention on Prohibitions or Restrictions on the Use of Certain Conventional Weapons (Protocol III).
However, one may first question the extent to which the flamethrower designed by Musk’s company qualifies as a proper incendiary weapon in the technical sense of the term. Indeed, as
noted by a commentator, the flamethrower is ‘more reminiscent of an oversized butane-jet lighter than the fiery liquid mixture of nitrogen propellant and gasoline that defined the [US] Army’s M2 flamethrower during the Vietnam War’.
Article 1 of Protocol III, while referring to flamethrowers as an example, defines incendiary weapons for the purpose of that treaty as
any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or combination thereof, produced by a chemical reaction of a substance delivered on the target (emphasis added).
The mere use of butane or other gas would not therefore be sufficient to make a device an incendiary weapon for the purpose of applying that treaty. In any case they would remain weapons using fire and heat to cause harm.
Beyond this question, it is key to highlight that the norms contained in Protocol III only refer to
limitations in use for incendiary weapons, and
not to a blanket prohibition in all circumstances. This is a transposition of the rules on the protection of civilians, such as the prohibition ‘in all circumstances to make the civilian population as such, individual civilians or civilian objects the object of attack by incendiary weapons’ (Article 2). It is uncontroversial that any use of a flamethrower, be it a proper incendiary weapon or not, in breach of those rules on the protection of civilians would be unlawful. The same would hold true for any weapon.
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