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Interactive simulation of battles is a valuable tool for training. The behavior and movement of hundreds or thousands of entities (tanks, trucks, airplanes, missiles, etc.) is currently simulated using dozens or more workstations on geographically distributed LANs connected by WANs. The simulated entities can move, fire weapons, receive "radio" messages, etc. The terrain that they traverse may change dynamically, for example due to rains turning dirt roads into mud or bombs forming craters. Thus the entities need to receive frequent information about the state of the terrain and the location and state of other entities. Typically, information is updated several times a second. As the number of simulated entities grows, the number of messages that need to be sent per unit of time can grow to unmanageable numbers. One approach to reducing the number of messages is to keep track of what entities need to know about which other entities and only send information to the entities that need to know. For example, tanks in Germany need not know about a change of course of a ship in the Pacific. This technique for reducing messages is known as interest management. Caltech and its Jet Propulsion Laboratory have implemented a simulation of this type on several large-scale parallel computers, exploiting both the compute power and the fast messaging fabric of such systems. The application is implemented using a heterogeneous approach. Some nodes are used to simulate entities, some to manage a database of terrain information, some to provide interest management functions, and some to route messages to the entities that do need to receive the information. Some of these tasks require more memory than others, some require faster processing capability.