When using a D-MILS approach for high-assurance systems it is often necessary to develop an assurance case, containing an argument supported by evidence, that demonstrates that the system has the required assurance properties (such as security or safety). In this paper, we describe our approach for developing a D-MILS assurance case, which is based upon a set of modular assurance case patterns that are automatically instantiated using a model-based instantiation process. We illustrate the application of our approach using a small cryptographic controller example and explain the benefits brought by our approach in support of DMILS.