Intervention, in international relations, generally refers to the dictatorial interference by a state or group of states in the internal or external affairs of another state, without its consent. This interference is typically coercive and aimed at influencing the domestic or foreign policy of the target state. Under the UN Charter (Article 2(4)), the use of force against the territorial integrity or political independence of any state is prohibited, and the principle of non-intervention in the domestic affairs of states is a cornerstone of international law. However, there are specific, highly debated exceptions and justifications for intervention.

Intervention on Humanitarian Grounds (Humanitarian Intervention): Humanitarian intervention refers to the use of military force by one or more states within the territory of another state, without the consent of its government, with the stated purpose of preventing or ending a mass atrocity crime (genocide, ethnic cleansing, war crimes, or crimes against humanity). This concept emerged prominently in the post-Cold War era, particularly in response to atrocities in Rwanda, Bosnia, and Kosovo.

• Legal and Moral Dilemma: Humanitarian intervention presents a profound tension between state sovereignty (the bedrock of international law) and the moral imperative to protect populations from mass atrocities. Critics argue it violates sovereignty and the UN Charter's prohibition on the use of force, potentially leading to selective application, abuse of power, and destabilization. Proponents argue that sovereignty entails responsibility, and when a state fails to protect its own population from mass atrocities, the international community has a responsibility to intervene.
• Responsibility to Protect (R2P): In 2005, the UN General Assembly unanimously endorsed the 'Responsibility to Protect' (R2P) doctrine. R2P asserts that states have a primary responsibility to protect their own populations from genocide, war crimes, ethnic cleansing, and crimes against humanity. If a state fails to do so, the international community has a responsibility to assist it. If the state manifestly fails to protect its population, the international community has a responsibility to intervene, using diplomatic, humanitarian, and other peaceful means, and as a last resort, collective military intervention authorized by the UN Security Council (UNSC). R2P aims to legitimize humanitarian intervention under specific conditions and with UNSC authorization, thereby addressing concerns about unilateral action.

Intervention Due to Self-Defence: Intervention based on self-defence is explicitly recognized as a legitimate exception to the prohibition on the use of force under international law, specifically in Article 51 of the UN Charter. This article states that nothing in the Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.

• Conditions for Legitimate Self-Defence: For an act of self-defence to be lawful, it must meet certain criteria:
  • Armed Attack: There must be an actual or imminent armed attack against the state. The definition of 'armed attack' is crucial and generally refers to significant use of force, not merely border skirmishes or minor incidents.
  • Necessity: The use of force must be necessary to repel the attack and there must be no other reasonable means available to address the threat.
  • Proportionality: The force used in self-defence must be proportionate to the armed attack and limited to what is necessary to repel it.
• Anticipatory/Pre-emptive Self-Defence: The concept of 'anticipatory self-defence' (responding to an imminent attack) is highly debated but generally accepted under strict conditions (e.g., 'Caroline doctrine' of necessity being instant, overwhelming, leaving no choice of means, and no moment for deliberation). 'Pre-emptive self-defence' (responding to a potential, but not imminent, future threat) is largely rejected under international law as it could justify aggressive wars.
• Collective Self-Defence: Article 51 also allows for collective self-defence, where one state can come to the aid of another state that has been subjected to an armed attack (e.g., NATO's Article 5). This requires a request from the victim state.

In summary, while intervention is generally prohibited, humanitarian intervention, particularly under the R2P framework with UNSC authorization, and intervention in self-defence against an armed attack, are recognized as exceptional circumstances. Both remain areas of complex legal and political debate, constantly evolving with global events and interpretations of international law.