Operational decision making is one of the foremost cognitive demands in the field for infantry. Thus, there is increasing emphasis on training that enables Warfighters to build adaptive operational decision making (ODM) skills. Assessment of the effectiveness of this training is critical for determining its value with respect to both the cost and impact on Warfighters' limited time. To this end, an assessment method was developed and evaluated during a series of studies conducted at the Camp Pendleton Infantry Immersion Trainer (IIT). The method includes pre- and post-training assessments to allow quantification of training-related changes in ODM. Twenty-five IIT training scenarios were decomposed to extract ODM training objectives, from which 26 decision areas were selected. The decision areas fall into four competency areas emphasized during IIT training (Combat Hunter/Every Marine a Collector skills; Communication; Cultural Interaction; Infantry Squad/Team Skills). Multiple decision dilemmas resembling situations that may be encountered during field operations were developed for each decision area. Five potential courses of action (COAs) were developed for each dilemma. An assessment consists of a series of items (dilemmas and COAs); trainees use a 5-point scale to rate the effectiveness of each COA for addressing the associated dilemma. As part of the item development process, the dilemmas and COAs were subjected to a two-stage verification and validation process by separate sets of infantry subject matter experts (SMEs) to ensure face validity and consensus of SME ratings of COAs. Trainees completed one assessment prior to training, and another with similar (but not identical) items immediately following training. Their ratings were compared to those of SMEs. This paper describes the approach in greater detail, along with preliminary assessment results and a discussion of factors contributing to ODM training effectiveness.