This paper describes an example problem of knowledge-based belief network construction (Goldman, Breese and Wellman, 1992; Goldman and Charniak, 1993; Laskey, 1990). Knowledgebased model construction requires declarative representations for encoding modular, abstract, repeatable domain relationships, and procedures for instantiating and combining these knowledge elements to form models for particular problem instances (Egar and Musen, 1993; Regan and Holzman, 1992). Recent work in knowledge representation (Laskey and Mahoney, 1997; Mahoney and Laskey, 1996; Koller and Pfeffer, 1997) represents domain relationships as fragments of belief networks. The object-oriented framework is natural for this purpose, with its ability to represent abstract types with associated structure and methods, inheritance, and encapsulation. The example problem is drawn from the domain of military situation assessment. Although highly simplified, the example problem illustrates many of the issues that must be tackled to apply the technology to more complex problems. We discuss roles for mixed-initiative reasoning in knowledge-based model construction for military situation assessment.