This paper reports on an ongoing effort to define the security architecture for the Joint Simulation System (JSIMS), a joint military modeling and simulation system. It also describes the use of the security architecture to support the accreditation of the system.The JSIMS security architecture must coordinate not only enclaves at different classifications, but also the independent configurations of the multiple stakeholders. These include the various military branches and their separate designated approving authorities.It has therefore been necessary to develop the security architecture with sufficient breadth and flexibility to describe a variety of JSIMS instantiations, allowing an integrated accreditation by the multiple authorities without necessitating entirely independent accreditations. We have addressed the objective of flexibility by establishing a base, logical architecture along with customized versions of the architecture to meet the joint security objectives.