World War I

Sailing for France in September 1917, Thomas saw action with the 6th Marines at Verdun, Belleau Wood, Soissons, and in the Meuse-Argonne Offensive (Champagne). He was promoted to sergeant and served as an intelligence sergeant. During the battle in Belleau Wood, he led a squad of Marines and received the Silver Star for bravery.^[2] At Soisson, his company sustained heavy losses and he became the acting platoon leader. In September 1918, he was commissioned a second lieutenant. During his World War I service, in addition to the award of the Silver Star for valor, he was awarded the Purple Heart for wounds received in battle, and as a member of the 6th Marine Regiment, he became entitled to wear the French Fourragere. After participating in the occupation of Germany, he returned to the United States in July 1919.

Haiti

In November 1919, he joined the 1st Provisional Marine Brigade in Haiti, taking part in action against Haitian bandit forces until May 1921. During this time, he participated in the Battle of Port-au-Prince (1920) where he successfully ambushed a column of Caco rebels.^[3] In August 1921, he was assigned to the Marine barracks at Quantico, where he remained for the next two years. During that time he was detached for several months of duty with the guard company at the Disarmament Conference in Washington. He also completed the Company Officers Course at the Marine Corps Schools.

1920s-1930s

From November 1923 to October 1925, Thomas commanded the Marine Detachment aboard the USS Tulsa. He was then stationed for two years at the Marine Barracks, Navy Yard, Charleston, South Carolina, and for a year at Camp Holabird, Maryland, where he completed a course in the Army Motor Transport School. In December 1928, after serving as Officer-in-Charge of Land Transportation at the Marine Barracks, Parris Island, South Carolina, he joined the 1st Brigade Marines in Haiti, and became aide to the commanding general. He returned to the United States in June 1931, entering the Army Infantry School at Fort Benning, Georgia, the following September.

Graduating in June 1932, Thomas was made an instructor in the Basic School at the Philadelphia Navy Yard. He remained there until August 1934, when he was ordered back to Quantico as a student in the Senior Course. After completing that course, he was ordered to China in July 1935, for duty with the Marine Detachment at the American Embassy in Peiping. He returned to the United States two years later to attend the Command and General Staff School at Fort Leavenworth, Kansas, and after graduating in June 1938, he joined the staff of the Marine Corps Schools at Quantico.

World War II

In May 1941, Thomas was transferred to Cairo, Egypt, as a naval observer, but he was recalled to Quantico two months later to become assistant operations officer of the 1st Marine Division. Named Operations Officer of the division in March 1942, he sailed for the South Pacific in that capacity two months later. He was awarded the Navy Distinguished Service Medal for his efforts in the assault and capture of Guadalcanal and Tulagi. He was appointed chief of staff of the division in September 1942, at Guadalcanal, and in July 1943, he became chief of staff of the 1st Marine Amphibious Corps, where he was awarded the Legion of Merit with Combat "V" in the Treasury-Bougainville operation. After participating in the Empress Augusta Bay operation at Bougainville, he returned to Marine Corps Headquarters in Washington, D.C., where he was made director of Plans and Policies in January 1944 and awarded a second Legion of Merit for outstanding service from January 1944 to November 1946.

Korean War

In July 1947, Thomas was named Commanding General of Fleet Marine Force, Western Pacific. After that unit was disbanded in March 1949, he became chief of staff of the Marine Corps Equipment Board at Quantico, later serving there as commanding general of the Landing Force Development Center.

He took command of the 1st Marine Division during the Korean War in April 1951, where he earned the Army Distinguished Service Cross and Army Distinguished Service Medal. He served in that capacity until January 1952, when he was relieved by Major general John T. Selden and returned to the United States. The following month he was promoted to the rank of lieutenant general and designated by President Truman as assistant commandant of the Marine Corps. He served in that billet until June 1954, and the following month became commandant of the Marine Corps Schools, Quantico.