The Army Battle Lab Collaborative Simulation Environment (BLCSE) operates a distributed laboratory network in support of Army Concept Development Experimentation Plan (ACDEP). While operating in an IEEE 1278 (DIS) technical mode, the BLCSE was required to become compliant to and interoperable with IEEE 1516 (HLA) federations, yet retain the ability to operate in DIS mode during the process, which would overlap two major experiments. Although use of DIS-HLA Gateways, on either a by-machine or by-location basis, was evaluated as feasible, cost, complexity, and latency concerns forced an embedded Middleware solution. This paper discusses the operating environment concerns that precluded the traditional Gateway interoperability, the development and integration of Middleware integrated into compiled executable software programs, and the management process that enabled the Middleware to maintain DIS functionalities while the disparate federation simulations and tools were enabled compliant to the HLA. Lessons learned on Middleware integration of DIS-HLA federates while making the system interoperable between two major operating environments are provided. Project development began in late FY05, and all federates will complete Middleware integration prior to the end of FY06.