1. Core Legal Answer & Context: The use of force in international relations is primarily governed by the UN Charter, which generally prohibits it, with two main exceptions: self-defence and Security Council authorization. Article 2(4) of the UN Charter prohibits the threat or use of force against the territorial integrity or political independence of any state.

Right of Self-Defence (Article 51 of UN Charter): This article preserves 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 Lawful Self-Defence:

• Armed Attack: A state must have suffered an actual "armed attack." The ICJ in the Nicaragua Case distinguished between the "most grave forms of the use of force" (armed attack) and other lesser forms. The concept of "anticipatory self-defence" (pre-emptive strike against an imminent attack) is debated, but generally requires a very high threshold of imminence (Caroline test: "necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation").
• Necessity: The use of force must be necessary to repel the armed attack. There must be no other reasonable means available to halt or repel the attack.
• Proportionality: The force used must be proportionate to the armed attack suffered. It should not exceed what is required to achieve the defensive objective.
• Reporting to UNSC: Measures taken in self-defence must be immediately reported to the Security Council.
• Temporary Nature: The right exists "until the Security Council has taken measures necessary to maintain international peace and security."

Collective Self-Defence: Article 51 also allows for collective self-defence, where a state can come to the aid of another state that has suffered an armed attack, provided the victim state requests assistance.

2. Relevant Statutes and Sections:

• United Nations Charter, Article 2(4) (general prohibition on the threat or use of force).
• United Nations Charter, Article 51 (explicitly preserves the inherent right of individual or collective self-defence).

3. Important Landmark Cases:

• Nicaragua Case (Nicaragua v. United States of America) (ICJ, 1986): The ICJ clarified the customary international law on the use of force and self-defence, distinguishing between an "armed attack" (triggering Article 51) and other forms of intervention. It affirmed the requirements of necessity and proportionality.
• Caroline Incident (1837): This incident, though not a judicial case, established the customary international law standard for anticipatory self-defence, requiring "necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation."

4. Clear Conclusion: International law, primarily through the UN Charter, strictly regulates the use of force, making self-defence the principal exception to the general prohibition. The exercise of this inherent right is subject to stringent conditions of an armed attack, necessity, proportionality, and reporting to the Security Council, reflecting the international community's commitment to limiting unilateral military action and maintaining international peace and security.