This paper is a case study of the British military intervention into Sierra Leone in 2000. The successful British intervention led to defeat of the Revolutionary United Front (RUF), final peace accords, and brought order to a failed state. The paper will explore the following points: what was the British foreign policy and what impact did it have in the decision to intervene; what was the British counterinsurgency (COIN) doctrine and was it useful for the forces in Sierra Leone; did the British forces use their own doctrine or was the situation in Sierra Leone unique; why was the intervention successful and what lessons can be drawn? Beyond the scope of this paper is a comparison of United Kingdom (UK) COIN doctrine and current United States (US) COIN doctrine. The focus will be to analyze the UK's actions against their doctrine, not the doctrine of the US. There is one major assumption for this case study. The paper categorizes the intervention into Sierra Leone as successful. The justification for the assumption is current day Sierra Leone. Instead of a war torn failed state, Sierra Leone has lasting peace, completed disarmament of insurgent forces, ended the large scale human rights abuse, and democratic elections, not coups, determining the leadership of the country.