This thesis examines the question of why and how countries that commit to a military intervention eventually decide to disengage. Its main hypothesis is that there is a relationship between the type of military setbacks experienced by policy-makers and the policy recommendations they subsequently put forward and adopt. Four military interventions - the U.S. intervention in Vietnam in 1967-1969, the U.S. intervention in Lebanon in 1982-1984, the French intervention in Lebanon in 1982-1984, and the U.S. intervention in Somalia in 1992-1993 - are examined in order to identify which events occurring during these interventions led policy makers to decide to disengage. Examination of twelve military setbacks over these four cases suggests that the surprise factor plays a particulary important role in the decision to disengage or not. Surprising setbacks tend to lead to disengagement of the intervening power, while non-surprising setbacks usually lead to limited escalatory moves