This paper summarizes our experiences in using computer-supported methods to develop a software architecture to support the re-engineering of the Janus Combat Simulation System. We have analyzed the Janus FORTRAN source code, interviewed Janus domain experts, developed an object-oriented architecture for the Janus Combat Simulation subsystem, and validated the architecture with an executable prototype. In this paper, we propose methods to facilitate the evolution of the software component of these systems by recovering the behavior of the systems using systematic methods, and illustrate their use in the context of the Janus System.