Over the past few decades, the Department of Defense (DoD) has invested millions of dollars into interoperability and reuse solutions for models and simulations. While some architectures and tools have been developed to address specific community needs, we do not yet have an overall common technical framework. The barriers range across the technical and social spectrum, from computing power and data compatibility, to funding and resistance to mandated architecture policy. Meanwhile, the DoD's evolving information technology (IT) infrastructure may soon provide the computing power, network, and storage necessary to make transition of modeling and simulation assets to service oriented architectures making composability both feasible and affordable. Maximizing the value of service based architectures while maintaining Joint and Coalition interoperability requires a coordinated effort, across the DoD and internationally. The Defense Modeling & Simulation Reference Architecture (DMSRA) will provide best practices and guidance to assist modeling and simulation (M&S) activities' use of Department of Defense (DoD) information technology (IT) and enterprise services, leveraging technologies such as cloud computing and service oriented architectures (SOA), through adoption of DoD M&S enterprise standards to facilitate sharing of model and simulation assets. This paper provides insights to manage modular, open architectures for defense M&S with a future focus of delivering these capabilities through service oriented architectures. It discusses the challenges in applying DoD guidance for reference architectures for the level of abstraction necessary for the scope of a reference architecture for DoD M&S. It describes the process used for developing the document through collaboration with the DoD Components and Services, and international community (e.g. North Atlantic Treaty Organization (NATO) Modeling & Simulation Group (MSG)-136), initially within the Training domain and expanding to include Acquisition and Test & Evaluation (T&E).