1. Core Legal Answer & Context: In international law, 'intervention' refers to the dictatorial or coercive interference by a state or group of states in the internal or external affairs of another state, aimed at compelling it to act in a certain way. The principle of non-intervention is a cornerstone of state sovereignty and is enshrined in the UN Charter, prohibiting states from interfering in matters essentially within the domestic jurisdiction of any state. However, states have historically invoked various, often controversial, grounds to justify intervention.

a) General Prohibition: Article 2(4) of the UN Charter prohibits the threat or use of force against the territorial integrity or political independence of any state, and Article 2(7) reinforces the principle of non-intervention in matters essentially within the domestic jurisdiction of any state.

b) Grounds for Justification (often debated and limited):

• Invitation by the Legitimate Government: Intervention is generally considered lawful if it occurs at the express invitation or with the consent of the legitimate government of the target state. This is common for military assistance against internal rebellions or external threats. The key challenge lies in determining what constitutes a 'legitimate' government, especially during civil conflicts.
• Self-Defense (Individual or Collective): Article 51 of the UN Charter preserves the inherent right of individual or collective self-defense if an armed attack occurs against a UN Member State. A state may intervene in another state's territory if that state is unwilling or unable to prevent armed attacks emanating from its territory against the intervening state or its allies. This justification is subject to strict conditions of necessity and proportionality.
• UN Security Council Authorization: This is the most legitimate and universally accepted ground for intervention. Under Chapter VII of the UN Charter, the Security Council can determine the existence of any threat to the peace, breach of the peace, or act of aggression and authorize measures, including military force, to maintain or restore international peace and security. Examples include interventions in Korea (1950), the Gulf War (1990-91), and Libya (2011).
• Humanitarian Intervention / Responsibility to Protect (R2P): This is a highly contentious ground. It posits that states have a right or even a duty to intervene in another state to prevent or halt mass atrocities (genocide, war crimes, ethnic cleansing, crimes against humanity) when the host state is unwilling or unable to protect its own population. The 2005 World Summit Outcome Document endorsed the 'Responsibility to Protect' (R2P) doctrine, stating that states have a primary responsibility to protect their populations, and if they fail, the international community has a responsibility to use appropriate diplomatic, humanitarian, and other peaceful means. If these are insufficient, and national authorities are manifestly failing, the Security Council may authorize coercive measures, including force, under Chapter VII. The controversy lies in the lack of a clear legal basis for unilateral humanitarian intervention outside UNSC authorization, concerns about selectivity, potential for abuse, and the tension with state sovereignty.
• Protection of Nationals Abroad: States have occasionally justified intervention to protect their own citizens in another state when their lives are in imminent danger and the host state is unable or unwilling to provide protection. This justification is subject to very strict conditions, including imminent threat, proportionality, and the absence of other means.
• Counter-Terrorism: In recent decades, states have sometimes justified interventions (e.g., drone strikes, special forces operations) in other states against non-state terrorist groups, often citing self-defense or implied consent from a weak host state. This remains legally debated, particularly regarding the scope of self-defense against non-state actors and respect for territorial sovereignty.

2. Relevant Statutes and Sections:

• UN Charter, Article 2(4): Prohibits the threat or use of force against the territorial integrity or political independence of any state.
• UN Charter, Article 2(7): Enshrines the principle of non-intervention in matters essentially within the domestic jurisdiction of any state.
• UN Charter, Article 51: Preserves the inherent right of individual or collective self-defense if an armed attack occurs.
• UN Charter, Chapter VII: Grants the Security Council powers to take action with respect to threats to the peace, breaches of the peace, and acts of aggression.
• 2005 World Summit Outcome Document: Endorsed the Responsibility to Protect (R2P) doctrine.

3. Important Landmark Cases:

• Corfu Channel Case (ICJ, 1949): The International Court of Justice rejected the UK's argument of 'self-help' or 'intervention' to clear mines from Albanian waters, affirming the principle of non-intervention and condemning the violation of Albanian sovereignty.
• Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States) (ICJ, 1986): The ICJ strongly reaffirmed the customary international law principle of non-intervention and rejected the US justifications for supporting the Contras, including collective self-defense without an armed attack by Nicaragua.

4. Clear Conclusion: While the principle of non-intervention is a fundamental norm of international law, states have historically sought to justify interventions on various grounds. The only universally accepted legal basis for coercive intervention is authorization by the UN Security Council under Chapter VII. Other justifications, particularly humanitarian intervention and the protection of nationals, remain highly controversial and are subject to strict legal interpretation and political debate, reflecting the ongoing tension between state sovereignty and evolving international responsibilities.