Whilst the impact of genocide on the populations being targeted is routinely studied, the impact of genocide on international society is routinely overlooked. With this in mind, this thesis brings the study of genocide into IR, via the English School, in order to understand the broader impact of genocide on the ordering structure of international society. The thesis puts forward a novel approach in that it explores the relationship between genocide and international legitimacy and how this relationship has critical implications for the United Nations. It will be argued that genocide holds a special relationship with international legitimacy because it is internationally regarded as the "crime of crimes" from both a legal and moral perspective. It is proposed therefore, that this particular injustice has more of a profound impact on the ordering structure of international society than is presently recognised. In sharp contrast to much of the thinking that underpins present foreign policymaking, it will be claimed that because of the special relationship that genocide holds with international legitimacy, genocide can be understood to pose a threat to international order as it erodes both the legitimate authority of the UN (which acts as the cornerstone of international legitimacy) and the UN Security Council (which acts as the stabilising function in international relations) more than any other crime. It is hoped that through understanding the crime's relationship with international legitimacy, and the post-Cold War legitimacy crisis, a more informed understanding of genocide can be acheived. Although the 2005 UN-led Responsibility to Protect initiative addressed some of the issues at hand, its endorsement has not resolved the fundamental problem of altering political will. If one accepts that genocide has a significant impact on international order, then one has to accept that the prevention of genocide is within the national interest of all states, that is, if they value international stability.