In October 1846, Charbonneau, Antoine Leroux and Pauline Weaver were hired as scouts by General Stephen W. Kearny. Charbonneau's experience with military marches, such as with James William Abert in August 1845, along the Canadian River, and his fluency in Indian languages qualified him for the position. Kearny directed him to join Colonel Philip St. George Cooke on an arduous march from Santa Fe, New Mexico, to San Diego, California, a distance of . Their mission was to build the first wagon road to Southern California and to guide some 20 huge Murphy supply wagons to the west coast for the military during the Mexican–American War.