[2] Rupertus began his military career immediately after graduating high school, serving in the District of Columbia National Guard from 1907 to 1910.

Rupertus served on the Marine Corps rifle team, earning the Distinguished Marksman badge and winning a number of shooting matches.

Rupertus was serving aboard the battleship USS Florida when the United States entered World War I and was subsequently recalled up to the U.S. to command a detachment of Marines headed for Port-au-Prince, Haiti.

Despite the comparatively short time afforded him in which to organize his command, Brigadier General Rupertus quickly and efficiently assembled a provisional staff, and with their aid, his forces landed on Tulagi, Gavutu and Tanambogo, British Solomon Islands, and successfully assaulted a series of strategically disposed and strongly defended enemy positions.

Personally conducting the operation and dauntlessly exposing himself to enemy fire whenever necessary, he displayed exceptional courage and cool determination which served as an inspiration to the officers and men of his command.

His bold and judicious decisions and his high professional attainments contributed effectively to the success of our operations in the Tulagi Area and his conduct throughout was in keeping with the highest traditions of the United States Naval Service.