This paper highlights the System Effectiveness methodology and its role in acquiring and sustaining U.S. military weapons systems from 1958 to 2021. Given the long period covered by this study and the many changes to the acquisition process, it would be reasonable to expect that the methodology would change and adapt, and the study supports this assumption. The study used three qualitative methods: a structured review of the literature related to System Effectiveness, a grounded theory analysis of the structured literature review, and a historiography of the initial grounded theory results. The research identified three epochs, the first two lasting approximately 22 years each. The conclusions are fourfold. First, the role of System Effectiveness today is vastly diminished from its original purpose because original material was not widely accessible to the community of interest during the formative years. Second, analysis of source documents provides insight into how to correct the misconceptions of the past and incorporate System Effectiveness into modern system engineering. Third, the models developed in epoch one may have relevance for today’s problems. The last conclusion is that an integrated research methodology is a valuable tool for making sense of a mass of conflicting information spread out over time. This the most comprehensive research on systems effectiveness ever published. Based on over 500 documents, it is rigorous and innovative and provides the basis for a structured and integrated approach for utilizing System Effectiveness in emerging U.S. defense system acquisition and sustainment challenges.