When the American Expeditionary Forces (AEF) was committed to combat in the spring of 1918 to halt a German advance towards Paris,[5] he participated in the Aisne-Marne offensive (Château-Thierry) where he was twice wounded in action at Belleau Wood during the fighting there in June 1918.

He returned to the front in August, rejoining the 5th Marines, and saw action in the St. Mihiel and Meuse-Argonne offensives (Champagne) where he was wounded for the third time, shot through the neck by a machine gun.

Shepherd returned to the States in December 1920, and was assigned as White House aide and aide-de-camp to the commandant of the Marine Corps, Major General John A. Lejeune.

After graduation Captain Shepherd was assigned overseas again, this time on detached duty with the Garde d'Haïti, serving for four years as a district and department commander in the United States occupation of Haiti.

In March 1942, four months after the United States entry into World War II, Colonel Shepherd took command of the 9th Marine Regiment.

In May 1944, Shepherd assumed command of the 1st Provisional Marine Brigade and led them in the invasion and subsequent recapture of Guam during July and August 1944.

[7] Several months later, Shepherd returned to the United States and in March 1946, organized the Troop Training Command, Amphibious Forces, Atlantic Fleet, at NAB Little Creek, Virginia.

When the Korean War erupted, Shepherd was in command of the Fleet Marine Force (FMF), Pacific, with headquarters at Pearl Harbor.

In this capacity, he played a major role in the amphibious assault at Inchon, earning a Silver Star,[7] and in the evacuation of U.S. forces from Hungnam following their withdrawal from the Chosin Reservoir in North Korea in December 1950.

During Shepherd's four years as commandant, he initiated a number of important policies that resulted in increased military proficiency for the Marine Corps, one of the first and widest reaching of which was the institution of a General Staff System.

During his three and a half years of service with this international organization, Shepherd, through his leadership and diplomacy, made substantial contributions towards plans for the defense of the continent.