Major General William H. Rupertus, who commanded the 1st Marine Division on New Britain, was born at Washington, D.C., on 14 November 1889 and in June 1913 graduated from the U.S. Revenue Cutter Service School of Instruction. Instead of pursuing a career in this precursor of the U.S. Coast Guard, he accepted appointment as a second lieutenant in the Marine Corps. A vigorous advocate of rifle marksmanship throughout his career, he became a member of the Marine Corps Rifle Team in 1915, two years after entering the service, and won two major matches. During World War I, he commanded the Marine detachment on the USS Florida, assigned to the British Grand Fleet.

Between the World Wars, he served in a variety of assignments. In 1919, he joined the Provisional Marine Brigade at Port-au-Prince, Haiti, subsequently becoming inspector of constabulary with the Marine-trained gendarmerie and finally chief of the Port-au-Prince police force. Rupertus graduated in June 1926 from the Army Command and General Staff College at Fort Leavenworth, Kansas, and in January of the following year became Inspector of Target Practice for the Marine Corps. He had two tours of duty in China and commanded a battalion of the 4th Marines in Shanghai when the Japanese attacked the city's Chinese defenders in 1937.

During the Guadalcanal campaign, as a brigadier general, he was assistant division commander, 1st Marine Division, personally selected for the post by Major General Alexander A. Vandegrift, the division commander, whom he succeeded when Vandegrift left the division in July 1943. Major General Rupertus led the division on New Britain and at Peleliu. He died of a heart attack at Washington, D.C., on 25 March 1945, and did not see the surrender of Japan, which he had done so much to bring about.