This paper discusses the application of assurance cases as a means of building confidence that the software design of a complex system of systems will actually meet the operational objectives set forth in the project's top-level requirements. The discussion will explain the reasons for selecting the assurance case approach and step through an example of how to apply the method in the described context. The challenges of applying the approach in a real setting will be considered, followed by conclusions. This paper will serve as an anonymous case study. At the close of the paper, readers will have learned how assurance cases have been used in real, non-trivial settings to solve the stated problem. They will have a blueprint for applying similar techniques to their own challenges, with the insight into potential pitfalls that will allow them to succeed.