In the past, the right to execute wars belonged to the sovereignty of countries. A war between two countries was similar to a duel between two people, and it did not violate the "International law". It was not until after the two world wars that the "Principle of prohibition of force" and the "System of collective security" were introduced through article 2, item 4 of the charter of the united nations, that war between nations was generally prohibited.
According to this system, disputes between states should be resolved peacefully, and the only legitimate war is to start a war if the un security council logo design first determines whether there is a "Threat to the peace", "Violation of the peace", or "Aggression" in accordance with article 39 of the charter. Exist. If one of them is certain to exist, the security council can decide to impose the "Sanctions by force" of article 42 (or the economic sanctions of article 41).
But article 42 resolutions on force sanctions are often paralyzed because of the veto power of the permanent members of the security council. Therefore, the justification of "War" outside the security council resolution becomes very important. These include: the right to self-defense in article 51 of the charter of the united nations (individual self-defense, collective self-defense, preventive self-defense, and pre-emptive self-defense), and other justifications recognized by state practice: "Intervention upon invitation", "Saving one's own nationals" ", "Humanitarian intervention".