Denver's increasing demand for water led to the Foothills environmental dispute in 1977. This dispute featured various forms of third party intervention. Representative Patricia Schroeder's failure to mediate the Foothills conflict and Representative Timothy Wirth's success are compared in terms of the disputants' key beliefs affected by the two self-appointed intervenors' actions. Using the technique of counterfactual case analysis, an exploration is made of a range of possible timing and ground preparation decisions. Although the particular circumstances of any dispute play a key role in its resolution, the proposed perspective extracts features that are general and therefore transferable to other contexts, thus enabling mediators to develop, transmit, and apply intervention skills. A sequence of diagnosis and action is studied in a theoretical framework in which mediators examine and alter four classes of disputant perceptions: available set of actions; class of possible consequences; likelihood of uncertain events and consequences of actions; and preferences over consequences. 27 references, 2 tables, 3 figures.