Even while on active duty, Diamond lived informally, often going hatless and wearing dungarees in open defiance of military dress regulations. (He even accepted one of his decorations in dungarees.) Self-confidence, even cockiness, was one of his outstanding characteristics. He considered anybody with less than ten years in the Corps a "boot." While he bawled out recruits who sometimes instinctively saluted him, he frequently failed, himself, to salute less than a field grade officer.
"Mr. Marine" itched for more action and he soon got it in Shanghai, with Company M, 3rd Battalion, 4th Marine Regiment. But the Sino-Japanese controversy, in Diamond's opinion, was "not much of a war," and on 10 June 1933, he returned to the United States, disembarking from the USS Henderson (AP-1) at Mare Island, California. By then he was a Gunnery Sergeant.
Diamond returned to Shanghai with his old outfit, the 4th Marines, ten months later; was transferred to the 2nd Marines in December, 1934; and returned to the States in February, 1937. Two years after his promotion to Master Gunnery Sergeant on 10 July 1939, he was assigned to the Depot of Supplies at Philadelphia to help design a new infantry pack.
Though not a "spit-and-polish" Marine, Diamond proved himself an expert with both 60- and 81mm mortars, his accurate fire being credited as the turning point of many an engagement in the Pacific during World War II. Among the many fables concerning his Guadalcanal service is the tale that he lobbed a mortar shell down the smoke stack of an off-shore Japanese cruiser. It is considered a fact, however, that he drove the cruiser from the bay with his harassing "near-misses."
After two months on Guadalcanal, physical disabilities dictated his evacuation by air against his wishes. He was moved to the New Hebrides and later to a hospital in New Zealand, where he somehow acquired orders to board a supply ship for New Caledonia. There a friend ordered him back to Guadalcanal — the supposed location of his old outfit. Upon his arrival, however, Diamond discovered that the 1st Marine Division had shipped out to Australia, a distance of over 1,500 miles. Diamond made the trip, without orders, by bumming rides on planes, ships and trains.
But Diamond was destined to see no more combat. On 1 July 1943, he disembarked from the USS Hermitage (AP-54) at San Pedro, California, and twelve days later was made an instructor at the MCRD Parris Island, South Carolina. He was transferred to Camp Lejeune on 15 June 1945, and joined the 5th Training Battalion with the same duties.
Although Diamond is sometimes referred to as "highly decorated", his only personal decoration was the Secretary of the Navy Commendation Ribbon, which later became the Navy Commendation Medal. Diamond's other awards include:
- the Presidential Unit Citation awarded to the First Marine Division for Guadalcanal
- the Marine Corps Good Conduct Medal, eight awards representing twenty-four years service
- the World War I Victory Medal, with four campaign stars (Aisne-Marne, St. Mihiel, Meuse-Argonne, and Defensive Sector), for service with the 6th Marine Regiment.
- the Army of Occupation of Germany Medal, for service in the Rhineland in 1919
- the Yangtze Service Medal, for service in China in 1927-32
- the American Defense Service Medal, for service 1939-1941
- the American Theater Medal, for service in the United States 1943-1945
- the Asiatic-Pacific Campaign Medal, with two campaign stars (Guadalcanal-Tulagi Landings and Capture and Defense of Guadalcanal), for service with the 2nd Battalion 5th Marine Regiment
Diamond was also entitled to the French Fourragere (Croix de Guerre 1914-1918) as a personal award, since he had participated in earning it with the 6th Marines