Modern medium and heavy duty Commercial Off The Shelf (COTS) diesel engines take advantage of state-of-the-art technologies to deliver excellent performance while meeting the most stringent emissions legislation. While some of these technologies offer significant advantages in terms of engine efficiency, performance and weight versus traditional military engines, others are driven purely by the need to meet emissions standards. In order to successfully adapt these COTS engines for military use and fuel (JP-8), the emissions-only systems must be removed and the engine recalibrated for maximum efficiency. The downsized, turbocharged engine would enable a simultaneous improvement in engine weight, performance and efficiency in one of the DoDís largest fleet of vehicles - High Mobility Multipurpose Wheeled Vehicle (HMMWV), when compared to the current configuration. This paper will illustrate how a modern diesel engine was quickly developed from COTS to military-ready configuration for potential use in programs like the High Mobility Multipurpose Wheeled Vehicle (HMMWV) recapitalization and repower programs. The conversion of a 2011 model year COTS engine is shown as a case study for defense application as funded by TARDEC.