Article 51 of the United Nations Charter recognizes the 'inherent right of individual or collective self-defence if an armed attack occurs.' This phrasing forms the core of the debate regarding anticipatory and pre-emptive uses of force.

Anticipatory Self-Defence refers to the use of force in response to an imminent threat of an armed attack, where the attack has not yet materialized but is clearly about to occur. The classic formulation for this concept comes from the 1837 'Caroline incident,' which established the criteria of 'necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation.' Proponents argue that in an age of modern weaponry and rapid attacks, waiting for an attack to 'occur' might be too late. However, critics emphasize the potential for abuse and the difficulty of objectively determining 'imminence.' International Court of Justice (ICJ) jurisprudence, notably in the Nicaragua case, has generally adopted a restrictive interpretation of Article 51, emphasizing that an armed attack must have actually occurred to trigger the right to self-defence.

Pre-emptive Self-Defence, on the other hand, involves the use of force against a potential, non-imminent threat, often to prevent a future capability or a distant threat from developing. This concept gained prominence with the 'Bush Doctrine' post-9/11, which argued for the right to act against threats before they are fully formed. This is far more controversial and widely considered illegal under current international law. It essentially allows a state to initiate hostilities based on perceived future threats, which directly contradicts the UN Charter's primary objective of prohibiting the unilateral use of force and maintaining international peace and security.

In conclusion, while a narrow interpretation of anticipatory self-defence, strictly adhering to the 'Caroline' criteria of absolute necessity and imminence, has some theoretical and historical support, it remains a highly contentious area of international law. Pre-emptive self-defence, which involves acting against non-imminent threats, is largely rejected as a legitimate justification for the use of force under Article 51 of the UN Charter. The prevailing view among most states and international legal scholars is that Article 51 primarily permits self-defence only after an armed attack has occurred, or in extremely rare and clearly demonstrable cases of an imminent attack that meets stringent criteria, to prevent the erosion of the prohibition on the use of force.