While serving as a second lieutenant he did duty at the various Marine Barracks in the eastern part of the United States and served on board the USS PINTA for three years. He was again at sea on the USS IROQUOIS at the time he was promoted to first lieutenant in September 1890.

After completing the second two years' tour of sea duty he served one year at the Marine Barracks, Navy Yard, Washington, D.C., and was then attached to a Marine guard at the World's Colombian Exposition at Chicago, where he remained until it closed. At that time, he resumed his regular duties at the Washington, D.C. Navy Yard.

In June, 1896, he again went to sea, this time aboard the USS VERMONT. He was transferred to the USS SAN FRANCISCO in December 1897 and to the USS NEW ORLEANS during the following April. While serving on that vessel during the Spanish-American War, he participated in several bombardments of the forts at Santiago, Cuba. He was advanced to Captain on August 11, 1898, and was transferred to the USS CHICAGO in November of that year.

General Barnett came ashore for duty at Marine Corps Headquarters in Washington, D.C. in May, 1901, and was promoted to Major shortly afterwards. During the following year he was given command of a battalion of Marines on the USS PANTHER, sent for duty on the Isthmus of Panama, where they protected American interests and guarded the railway transit of the Isthmus.

General Barnett returned to Washington in December, 1902, only to be placed in command of another battalion of Marines being transferred less than a month later to join the First Brigade of Marines in the Philippine Islands. Arriving in the Philippines a few months later, he was transferred to duty as Fleet Marine Office of the Asiatic Fleet and served on several vessels of that fleet until December, 1904, when he rejoined the First Brigade Marines.

The general was transferred from the Philippine Islands to Washington, D.C. in April, 1905, and shortly after arriving in the United States, received a promotion to Lieutenant Colonel. He served as Commanding Officer, Marine Barracks, Navy Yard in Washington for a period of one year, when he was again placed in command of an expeditionary battalion which sailed on bard the USS MINNEAPOLIS for Havana, Cuba, where it landed and became part of the Cuban Army of Pacification.

General Barnett's organization was augmented to a regiment soon after landing in Cuba, while the entire Marine Expeditionary Force was increased to a brigade under the command of Colonel L.W.T. Waller. General Barnett's regiment was almost immediately transferred to Cienfuegos, where it spread out over a wide area with Barnett controlling a considerable portion of the island. A large army expeditionary force relieved part of the Marines in Cuba and General Barnett returned to Washington early in November, 1906.

After commanding the Marine Barracks in Washington for a period of one year, he was transferred to Marine Corps Headquarters and was shortly afterwards ordered to command the Marine Detachment, American Legation, Peking, China. Upon completing his tour of duty in the Far East, he returned to the United States during the summer of 1910 and assumed command of the Marine Barracks, Philadelphia, Pennsylvania. He was promoted to the rank of Colonel on October 11, that year.

During the next three years he was sent to Cuba each year in command of the First Regiment of Marines, which was repeatedly sent to that troublesome island on account of serious domestic disturbances which the United States was obligated to control under provisions of the Platt Amendment. While this serious undertaking was being conducted, the First Advance Base Brigade of Marines was organized at Philadelphia, under the command of General Barnett. He went with that organization on extensive maneuvers with the Atlantic Fleet to Puerto Rico from which he returned on February 15 and was appointed Major General Commandant of the Marine Corps on February 21, 1914 for a period of four years from February 25, 1914. He was the first Commandant to be appointed on a four-year detail in accordance with a law passed the previous year.

General Barnett had received his early military training at the U.S. Naval Academy and this early contact with the Navy, together with his many years at sea, placed him in an advantageous position which he skillfully used to draw the Marine Corps into closer and more cordial relations with the Navy. His tour as Commandant, which was extended by a second detail, was doubtless a more eventful period in the history of the Marine Corps. In addition to World War I, several important expeditions to countries in the Caribbean area took place.

The first important military event of his administration as Commandant was the sending of a reinforced brigade of Marines to take part in the operations which occurred at Vera Cruz, Mexico, during 1914. A minor intervention in Haiti was made during that year and an expeditionary force of Marines was kept afloat for some time along the west coast of Mexico. Serious trouble began to brew in both Haiti and Santo Domingo and within a year, it was necessary for the Marine Corps under General Barnett's guidance to place a brigade of Marines in each of these two countries, where they continued on duty until after the close of General Barnett's administration.

The World War activities of the Marine Corps, which were carried on under the general direction of General Barnett, are too far-flung, complicated and well-known for detailed mentioning in this brief history. The Marine Corps expanded to more than three thousand officers and approximately 75,000 enlisted men. In addition to continuing the occupation of Haiti and Santo Domingo and reinforcements of the regular stations of the Marine COrps, two brigades of Marines were sent to France, occupied parts of Cuba, while another was held in reserve at Galveston, texas and large training centers were maintained at Quantico, Virginia and Parris Island, South Carolina.

General Barnett not only saw the Marine Corps through World War I, but also through the difficult period of demobilization and reorganization at the close of the war. Throughout the entire war period, General Barnett proved to be a proud and capable leader. For his outstanding service he was honored by the French Government by being made a commander of the Legion of Honor, and he was awarded the Distinguished Service Medal by the Secretary of the Navy.

--- General Barnett was reappointed as Commandant in 1918 despite Secretary of the Navy Josephus Daniels' belief that no one should serve more than four years in the nation's capital. However, the exigencies of the war demanded personnel stability at the top. But in the process of gaining his reappointment, Barnett alienated the Secretary of the Navy.

Asked to sign an undated letter of resignation, Barnett refused. Since all officers serve at the pleasure of the President, he need only indicated when Barnett was no longer wanted. Although not pleased with such a response, Daniels retreated from his position but insisted later that he had Barnett's promise to resign when the war was over.

Also contributing to General Barnett's demise was his attempt to gain promotion to lieutenant general for the Commandant. In the spring of 1918, Barnett's Republican supporters in the Senate attached such a proviso to the Naval Appropriations Bills. Armed with knowledge that Daniels had admonished Barnett for going behind his back to gain the promotion, Congressman Thomas Butler, chairman of the House Naval Affairs Committee and father of Brigadier General Smedley D. Butler, rose in the House to denounce the promotion. In the process, Barnett's professional character was impugned.

Irritated by Barnett's pursuit of honors and promotions for the Headquarters staff in recognition of its wartime service, Daniels and Butler pressured Barnett into resigning as Commandant in June 1920, two full years before the expiration of his second term. Daniels had already decided that Barnett would be replaced by John A. Lejeune, which made way for Smedley Butler to be promoted to major general and to assume command at Quantico, the Corps' most important post.

General Barnett was relieved as Commandant of the Marine Corps on July 1, 1920, by order of the Secretary of the Navy. Surprisingly, he opted to remain on active duty while his political supporters moved to block Lejeune's confirmation.

He was made a Brigadier General in the Marine Corps as of August 29, 1916, while serving as Commandant, and on March 5, 1921, he was given the regular rank of Major General. He spent the remaining years of his active service as Commanding General of the Department of the Pacific.

General Barnett retired on December 9, 1923, having reached the statutory age limit of sixty-four years. He died on April 27, 1930 and was buried in Arlington National Cemetery, Arlington, Virginia.