Carl enlisted into Company F of the 4th Wisconsin Infantry Regiment (National Guard) in April 1896 and volunteered for service in the Spanish–American War in 1898. Brosius served in Company M of the 1st Wisconsin Infantry Regiment during the War, and was honorably discharged in August 1900. 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.