The Air Force Research Laboratory is conducting research and development of a computer-based simulation capability to support training in decision-making and team coordination for security forces ground operations. Simulation software supports the interaction (over a local area network) of trainees with each other and with computer-generated forces (CGF's) that simulate behavior and communications of enemy, neutral, and friendly troops and civilians. Radio functions allow multi-channel communication among instructors, trainees, and CGF's. Trainees can practice decision-making and team coordination in a number of scenarios with varying threat and environmental conditions. Current systems are too costly for training large numbers of security forces because they require an on-site technician to develop simulation exercises, control the exercise, serve as role players, task CGF's, and support after action reviews. Consequently, design and development of a simulation control interface that can be directly used by instructors and trainees is an important R&D objective. For the security forces simulation capability the goal is to design a control interface that instructors can learn to use in two hours and trainees can learn to use in thirty minutes. To achieve this goal, a Windows-based control interface (with a number of video game features) was adopted as the initial point of departure. Menu options were developed to correspond to the standard mission planning procedures used by security forces and drag-and-drop functions were developed to replace menu options to contribute to usability. We have conducted an evaluation of the initial user interface with school instructors and trainees. The paper describes the emerging control interface, the approach to and outcomes from a field evaluation of the interface to include actual times required for instructors and trainees to learn to use the system and instructors' acceptability evaluations. Lessons learned shed light on critical human-machine interface design issues for computer-based real-time training simulations.