 His father, George, served in the 35th Wisconsin Infantry Regiment during the Civil War and became a prominent physical trainer in the Milwaukee area.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.