Chapter VII of the United Nations Charter

Chapter VII of the United Nations Charter sets out the UN Security Council's powers to maintain peace. It gives the Security Council the authority to "determine the existence of any threat to the peace, breach of the peace, or act of aggression" and to take military and nonmilitary action to "restore international peace and security.”

Rationale

The UN Charter's prohibition of member states of the UN attacking other UN member states is central to the purpose for which the UN was founded in the wake of the destruction of World War II: to prevent war. This overriding concern is also reflected in the Nuremberg Trials' concept of a crime against peace "starting or waging a war against the territorial integrity, political independence or sovereignty of a state, or in violation of international treaties or agreements", which was held to be the crime that makes all war crimes possible.
Chapter VII also gives the Military Staff Committee responsibility for strategic coordination of forces placed at the disposal of the Security Council. It is made up of the chiefs of staff of the five permanent members of the Council. Otherwise, that chapter is used when the UNSC is authorizing either a member state or a coalition of the willing to act nationally or through regional organizations to address this threat – if necessary with all necessary measures, including the use of outright force. The phrase ‘all necessary measures’ is to be taken literally. Any military action performed through land, air, and sea forces is specifically allowed. Such action could entail troop deployment, the enforcement of a no-fly-zone, even the use of aerial bombardment.

Historical background

The United Nations was established after World War II and the ultimate failure of diplomacy despite the existence of the League of Nations in the years between the First and Second World War. The Security Council was thus granted broad powers through Chapter VII as a reaction to the failure of the League. These broad powers allow it to enjoy greater power than any other international organization in history. It can be argued that the strong executive powers granted to it give it the role of 'executive of the international community' or even of an 'international government'.
The covenant of the League of Nations provided, for the first time in history, enforcement of international responsibilities through economic and military sanctions. Member states were also obliged, even without prior decision by the council to take action against states that acted unlawfully in the eyes of the Covenant. This meant that the peace process was largely dependent on the willingness of member states, because the Covenant did not provide binding decisions; The Council of the League was only responsible for recommending military force. As well as this, Article 11 paragraph 1 of the League’s Covenant states:
This can be seen as an authorization of the use of force and other enforcement measures, however, states repeatedly insisted that this did not make decisions by the League binding.
This resulted in an unprecedented will by both the powers at the Dumbarton Oaks Conference and the states present at the San Francisco Conference to submit to a central organ like that of the Security Council. Despite long debate over whether the General Assembly should also have power over decisions made by the Security Council, it was eventually decided by a large majority vote that the Security Council should maintain its executive power because, as the major powers emphasized, a strong executive organ would be needed for the maintenance of world peace. This emphasis was advocated in particular by the Chinese representative, recalling the powerlessness of the League during the Manchuria Crisis.

Articles 41, 42, 43, and 44

Articles 41 and 42 jointly establish the right of the Security Council to arrange for the use of both non-armed and armed measures to put its decisions into effect.
Article 41
Article 42
Article 43
Article 44
In 1947, the Military Staff Committee reported its recommendations to the Security Council regarding the implementation of Article 43. The Chiefs of Staff of the Permanent Members, split by Cold War politics, could not agree. Of the 41 articles in the Report of the Military Staff Committee only 25 could be agreed between the five powers. Thus Article 43 agreements have never been concluded. However, the Security Council remained technically seized of the matter until 1997.

Chapter VII Resolutions

Most Chapter VII resolutions determine the existence of a threat to the peace, a breach of the peace, or an act of aggression in accordance with Article 39, and make a decision explicitly under Chapter VII. However, not all resolutions are that explicit, there is disagreement about the Chapter VII status of a small number of resolutions. As a reaction to this ambiguity, a formal definition of Chapter VII resolutions has recently been proposed:
Chapter VII resolutions are very rarely isolated measures. Often the first response to a crisis is a resolution demanding the crisis be ended. This is only later followed by an actual Chapter VII resolution detailing the measures required to secure compliance with the first resolution. Sometimes dozens of resolutions are passed in subsequent years to modify and extend the mandate of the first Chapter VII resolution as the situation evolves.
The list of Chapter VII interventions includes:

See also Timeline of United Nations peacekeeping missions, some of which were created under the authority of Chapter VI rather than VII.
Article 53 in Chapter VIII of the United Nations Charter permits "the Security Council to use regional arrangements as appropriate or to authorize enforcement action by such arrangements".

Article 51: Self-defence

Article 51 provides for the right of countries to engage in self-defence, including collective self-defence, against an armed attack and was included during the San Francisco Conference in 1945.

Usage by sovereign countries

According to a study by researchers at Harvard Law School, between 1945 and 2018, UN Member States submitted 433 communications to the Security Council of measures taken in purported exercise of the right of self-defense.
This article was the impetus for much international pact-making and has been cited by the United States as support for the Nicaragua case, the 2003 invasion of Iraq, and the legality of the Vietnam War, as well as by many other countries. According to that argument, "although South Vietnam is not an independent sovereign State or a member of the United Nations, it nevertheless enjoys the right of self-defense, and the United States is entitled to participate in its collective defense." Another aspect is if the right of self-defense still exists if the UN Security Council has taken measures to deal with the conflict. There are contradictory opinions whether this right still exists once the Security Council has taken action. Article 51 has been described as difficult to adjudicate with any certainty in real-life.
In a letter to the UN Security Council requesting military intervention in Yemen, Yemen's President Hadi invoked Article 51.
The United States used Article 51 to justify the assassination of Qasem Soleimani and U.S. airstrikes in Iraq and Syria against an Iran-backed militia group.
The president of Russia, Vladimir Putin, cited Article 51 in a speech to justify the 2022 invasion of Ukraine and escalation of the war in Donbas.