Strong consensus exists among the services that increasing the availability of live, virtual, and constructive integrated Training Capabilities (TCs) is a desirable strategy for accelerating unit readiness, while reducing overall training costs. The USMC is planning to implement the Squad Immersive Training Environment (SITE) which will be a home-station capability for: training squad core competencies under realistic conditions to pre-specified standards; being reconfigurable to support scenarios across the range of military operations; and conducting effective after-action reviews. An Analysis of Alternatives (AoA) study was conducted to evaluate existing USMC TCs in the context of ten materiel requirements that had been identified by a capabilities-based assessment. A significant challenge was determining the capability of existing TCs to meet the new SITE requirements in order to identify cost effective technical improvements. This paper describes an innovative capabilities-based assessment tool that was developed during the AoA to address this challenge. We describe how the SITE Assessment Method was developed and applied; how the findings led to a concept for employing SITE to build squad task proficiency; and how to develop a rationale for making decisions about TC integration and interoperability. Lessons learned and guidelines are provided for conducting an AoA for unit training systems.