Civil wars occur in some countries at some times and not in other countries at other times. This articleexamines how the potential for large-scale external intervention can prevent civil wars. The authorargues that intervention by external states in civil war can be so overwhelming that it reduces one side’s probability of victory to essentially zero. When dissidents expect this type of intervention on the side of government, they anticipate no chance of achieving success through violence and do not initiate civil wars. When governments anticipate this type of intervention on their behalf, they feel protected from internal threat and are less constrained in their dealings with their populations. This repression increases grievances, leading dissidents to engage in strategies of dissent other than civil war. The authortests three implications of this argument-that states in more hierarchical relationships will experience civil war at lower rates, be more repressive, and experience other forms of dissent at higher rates-and finds strong support for it.