Presented in this paper is architecture of a simulation system for the military engagement. The architecture proposed in this paper is for the construction of a simulation system based on agent simulation design methodology. Every entity of the proposed architecture is defined of assemblage of modularized agent components, and it enhances the reusability and composability of the entity. Consequently, time, costs, and efforts that are required to develop a new simulation system are able to be reduced by the improved entity modeling methodology of the architecture. In the case of the military engagement simulation, reflection of environmental effects to simulation entities is very important, because real battle entities are operating in the battlefield of which cannot be controlled by operators. The proposed architecture contains the synthetic battlefield to simulate interaction between the environment and simulation entities. Thus, each behavior of simulation entities is changeable by the effects, and it can derive more precise results that well-represent real system operation. In conclusion, the proposed architecture supports swift construction of various engagement simulation systems that are based on the synthetic battlefield. The architecture of this paper is implemented to construct an example system for verification of the advantages.