Article 2, paragraph 4 of the UN Charter stipulates that: "All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations." This provision essentially outlaws the use of force by one State against another, except in very specific circumstances spelled out elsewhere in the Charter (self-defense and enforcement measures sanctioned by the Security Council). Employing mercenaries to use force against another State comes within the scope of this prohibition.
https://www.ohchr.org/en/node/3383/international-standards
I don’t know much about the legal technicalities of how the UN would set in motion a trial process if they suspect a nation broke a law in the UN Charter. Does a plaintiff have to submit a complaint at the international court?
I am curious if the question of whether the US invasion of Iraq was illegal according to the UN Charter is something that is still “open to interpretation” by a judge, or if there were any rulings by an international court that they violated Article 4, Paragraph 2?