This article considers the ways in which the role of humanitarian intervention, which currently does not have any clear legal status, can be defended through international law. Using both realist and liberal cosmopolitan theoretical approaches, it considers three notions of legitimacy to argue that humanitarian intervention can be justified: legitimacy through humanitarian crises; humanitarian intervention supported through a creative interpretation and application of general principles of international law; and humanitarian intervention justified by morality and “natural law.” It considers the use of force that has taken place in Iraq, Afghanistan, and Libya as case studies and examples. A broader sense of legality is argued to reconcile the notion that humanitarian intervention could be legalized; however, a restrictive factor remains in the horizontal rather than vertical nature of international law, which requires the will of States. Thus, the next best alternative is considered. Humanitarian intervention both before and after the adoption of the UN Charter has not gained the status of established international law sufficient to justify the use of force. From a purely legalist perspective, it is not permitted. Nevertheless, there is support for the concept. In the present day, one might assume that the use of force for humanitarian purposes remains subject to each State’s understanding of a particular situation. Therefore, it is important to consider the legitimacy of “non-legal” action. To consider any use of force legitimate, it must be perceived by other States as legitimate; therefore, it must be widely accepted as appropriate conduct. Usually, a State embarking upon the use of armed force will search widely for a legal justification, even if the argument is weak, in order to obtain support for the action. In Iraq (2003), the coalition sought to base its action on the ‘legality’ of UNSC-adopted Resolutions. In Afghanistan (2001), the USA based its argument on the ‘legality’ of self-defence. Council inaction has also been suggested as an argument for “non-legal” intervention. If the institution responsible for maintaining peace and security fails to fulfil its function, then a State may be justified, from a realist and practical point of view, in taking unilateral steps. Nevertheless, such action would still not be strictly ‘legal’ and could therefore be considered potentially ‘illegal’ under international law. It could be argued that the failure of the UNSC to condemn what is, from a strictly positivist standpoint, ‘illegal’ action, such as that in Kosovo (1998–99), gives it some legal status with possible precedential value. If this is so, then the UNSC could potentially authorize the use of force retroactively, including armed force, provided that it remains within the spirit of the relevant Resolution. However, this view is problematic because it undermines the rule of law. In order to follow the law, one must know what the applicable rule is at the time the transgression takes place. Otherwise, it would be impossible to know what the law was at any particular point in time and, therefore, impossible to determine whether a transgression had occurred. Moreover, failing to condemn a use of force is not necessarily equivalent to condoning it or rendering it legal. Perhaps a more suitable characterization is that such use of force is ‘tolerated.’ Legalists would not recognize any retroactive legality and would not consider legitimacy a viable alternative. It would require a form of law to set aside the UN Charter. However, from a realist and, even more so, a cosmopolitan perspective, there may be broader justifications or legitimate reasons for engaging in uses of force that are strictly illegal. Nevertheless, these arguments concern legality in a broader sense. They do not always conform to a strict understanding of legality or legalist theory. Kohen even suggests that US policies were not concerned with strict legality. If this were so, then even legitimacy would constitute a weak argument if the reasons for intervention were purely nationalistic interests. Nevertheless, the USA, and any State engaging in the use of force, should be required to justify its decision to the international community of States, including the UN. Therefore, whether legal or not, there must be a reason for each specific use of force. The following section considers various conceptions and models of legitimacy that could potentially provide a justification for humanitarian intervention that has not yet gained the status of international law *per se*.