What international law means that an individual who votes in favour of acquiring nuclear weapons, or who takes part in decisions to acquire, design, construct, deploy, maintain, or use nuclear weapons is taking on personal individual liability for possible crimes under international law and may be indicted before any competent legal tribunal (national or international)?
The source of the relevant international law is the Nuremberg Principles.
The Nuremberg Principles of International Law
Any person who commits an act which constitutes a crime under international law is responsible therefore and liable to punishment.
The fact that internal law does not impose a penalty for an act which constitutes a crime under international law does not relieve the person who committed the act from responsibility under international law.
The fact that a person who committed an act which constitutes a crime under international law acted as Head of State or responsible government official does not relieve him from responsibility under international law.
The fact that a person acted pursuant to order of his government or of a superior does not relieve him from responsibility under international law, provided a moral choice was in fact possible to him.
Any person charged with a crime under international law has the right to a fair trial on the facts and law.
The crimes hereinafter set out are punishable as crimes under international law:
a. Crimes against peace:
i Planning, preparation, initiation or waging of a war of aggression or a war in violation of international treaties, agreements or assurances;
ii Participation in a common plan or conspiracy for the accomplishment of any of the acts mentioned under (i)
b. War Crimes:
Violations of the laws or customs of war which include, but are not limited to, murder, ill-treatment of prisoners of war or persons on the seas, killing of hostages, plunder of public or private property, wanton destruction of cities, towns, or villages, or devastation not justified by military necessity.
c. Crimes against humanity:
Murder, extermination, enslavement, deportation and other inhuman acts done against any civilian population, or persecutions on political, racial or religious grounds, when such acts are done or such persecutions are carried out in execution of or in connection with any crime against peace or any war crime.
Complicity in the commission of a crime against peace, a war crime, or a crime against humanity as set forth in Principle VI is a crime under international law.