BACKGROUND: Military personnel are exposed to a high level of operational stress that degrades their psychophysiological and cognitive performance and could impact the mission. Commando training is a unique opportunity to assess psychological, physiological, and cognitive performance in an ecological setting. METHODS: Psychological, physiological, and cognitive performance were evaluated at baseline, and before and after a 1-week commando training course consisting of exercises and night walks. Psychological factors (anxiety, subjective stress), physiological measures (heart rate variability, electrodermal conductance), cognitive behavior (cognitive bias, risk-taking, decision-making), and cognitive performance (the MindPulse neurophysiological test) were assessed. Volunteers were 39 young (mean age 21.6 years) French army officers. RESULTS: Blind spot bias was found in all participants before training, except for the action-inaction bias. We observed a deterioration in parasympathetic functioning (Root Mean Square of Successive Differences, P = .05), an increase in subjective fatigue (P <
.001), and impaired cognitive performance: reaction time (P = .02, F = 5.77), errors (P = .03, F = 4.97). Post-training, we observed an emerging group dynamic, notably an increase in avoidance (buck-passing) (P = .002, F = 10,43), a reduction of the action-inaction bias (P = .009, F = 7.59), ostrich effect (P = .008, F = 7.83) and stereotyping bias (P = .03, F = 5.11). CONCLUSION: Commando training increases stress and impacts the cognitive performance of military personnel. Pre-deployment preparation could consider the need for physiological recovery, and the impact of deployment on cognitive performance.