It is widely accepted in international law that the responsibility of states for the breach of an obligation to prevent a given event occurs when that event takes place. When applied to the obligation to prevent genocide, it would mean that the breach of that obligation commences when genocide occurs. If that is the case, this would appear to be in conflict with the aim of the rules on the responsibility of states and the aim of the primary rule on the obligation to prevent genocide. This conflict has not been fully addressed in the literature. This contribution examines whether and to what extent under the existing rules of international law on the prevention of genocide it is possible to hold territorial states responsible for the breach of that obligation before genocide occurs or never occurs. It will be argued that international law does not provide a general rule which is applicable to all obligations to prevent and that the moment of the breach depends on the content and nature of each obligation and the event to be prevented. The nature and content of the obligation to prevent genocide dictates that its breach commences at the moment a state fails to do what that obligation requires it to do and not at the moment genocide is committed. The legal consequences of state responsibility for not taking measures to prevent genocide as well as the invocation of the responsibility are addressed, leading to the conclusion that an international mechanism seems to be required to effectively implement the responsibility of the territorial state to prevent genocide.