 In July 1913, he re-enlisted into Company D of the 1st Wisconsin Infantry Regiment; he was transferred to Company F in April 1915 and was promoted to 2nd Lieutenant two months later.Brosius was mustered into service for the Mexican Border War in June 1916, where he commanded an infantry regiment.Having earned a sterling reputation as a physical trainer, Brosius remained stateside during World War I, going from camp to camp to implement training programs for American soldiers.