Casus Belli

The casus belli is the justification for going to war. For the 193 countries that signed the UN Charter there are three reasons. Self defence sounds reasonable. Mutual defence ditto. UN authorization maybe, just maybe. That depends on politics, especially those of the Security Council which means China, France, Russia, the United Kingdom, the United States and ten temporary members. The Nuremberg Principles include the "planning, preparation, initiation or waging of a war of aggression or a war in violation of international treaties, agreements or assurances". These are more restrictive.

Casus Belli ex Wiki
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Casus belli is a Latin expression meaning the justification for acts of war. Casus means "incident", "rupture" or indeed "case", while belli means bellic ("of war"). It is usually distinguished from casus foederis, with casus belli being used to refer to offenses or threats directly against a nation, and casus foederis to refer to offenses or threats to another, allied, nation with which the justifying nation is engaged in a mutual defense treaty, such as NATO.

The term came into wide usage in the seventeenth and eighteenth centuries with the writings of Hugo Grotius (1653), Cornelius van Bynkershoek (1707), and Jean-Jacques Burlamaqui (1732), among others, and the rise of the political doctrine of jus ad bellum or "just war theory". Informal usage varies beyond its technical definition to refer to any "just cause" a nation may claim for entering into a conflict. As such, it has been used both retroactively to describe situations in history before the term came into wide usage and in the present day when describing situations when war has not been formally declared.

Formally, a government would lay out its reasons for going to war, as well as its intentions in prosecuting it and the steps that might be taken to avert it. In so doing, the government would attempt to demonstrate that it was going to war only as a last resort (ultima Ratio) and that it in fact possessed "just cause" for doing so. In theory international law today allows only three situations as legal cause to go to war: out of self-defense, defense of an ally under a mutual defense pact, or sanctioned by the UN................

Countries need a public justification for attacking another country. This justification is needed to galvanize internal support for the war, as well as gain the support of potential allies.

In the post World War Two era, the UN Charter prohibits signatory countries from engaging in war except 1) as a means of defending themselves against aggression, or 2) unless the UN as a body has given prior approval to the operation. The UN also reserves the right to ask member nations to intervene against non-signatory countries which embark on wars of aggression. In effect, this means that countries in the modern era must have a plausible casus belli for initiating military action, or risk UN sanctions or intervention.
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Public justifications? They did not bother when it came to attacking Libya, at least just barely. The claim that Colonel Gaddafi was not a nice chap sits ill with the fact that our lot were willing to do business with him. In fact they were down right eager. Human rights violations came absolutely nowhere. They even gave him the man alleged to have done the Lockerbie Massacre. Justice had nothing to do with it. Oil mattered. The rest was flim flam.

 

War Crime Defined By George Monbiot
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The Nuremberg Principles , which arose from the prosecution of Nazi war criminals, define as an international crime the "planning, preparation, initiation or waging of a war of aggression or a war in violation of international treaties, agreements or assurances". Bolton appears to have "participated in a common plan" to prepare for the war (also defined by the principles as a crime) by inserting the false claim that Iraq was seeking to procure uranium from Niger into a state department fact sheet.
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Mr. Monbiot is a peace campaigner among other things. He has principles [ sometimes ] and does something about them. His definition covers Blair, Brown, Bush, Cameron, Clinton, Adolf, Stalin, Benito Mussolini, Lenin, Netanyahu, Obama among others.