The management of large-scale incidents, such as demonstrations that can affect public order, requires complex decision making. In association with the London Metropolitan Police a computer-based simulation (CACTUS) was designed for improving the strategic and tactical management of public order events by senior police officers. It incorporates a digitized map with active (iconized) police, crowd and hostile agents able to navigate the map and interact autonomously in ways that simulate aggression and disorder if the police resources and their instructions are not managed with some skill. Adaptive training scenarios were designed in CACTUS by the trainer/facilitators covering planning, event management and debriefing. An evaluation study collected audio and video records of the training sessions and these data gave useful insights into the decision-making processes and how the CACTUS simulation, through its design features, became a dynamic mediational tool in developing such skills.