In January 1918, he sailed for France, and as a platoon leader in the 6th Marine Regiment, participated in the Aisne-Marne Defensive (Chateau-Thierry), where he was wounded in action; Belleau Wood; Bouresches; Soissons; and the St. Mihiel Offensive, where he was again wounded in action. He was evacuated to the United States in October 1918 for hospitalization.

For bravery in action, he was awarded the Silver Star Medal, was cited by the Commander-in-Chief, AEF, and in General Orders of the War Department, and entitled to wear the French Fourragere as a member of the 6th Marine Regiment.

Following recruiting duty in Kansas City, Missouri, he was assigned foreign shore duty with the 1st Provisional Marine Brigade in Haiti; sea duty aboard the USS Olympia; and again foreign shore duty, with the 2d Marine Brigade in Santo Domingo. In September 1924, he became Depot Quartermaster, Marine Barracks, Quantico, Virginia. He later completed instruction at the Army Infantry School, Fort Benning, Georgia, and was assigned to the Marine Corps Schools, Quantico as an instructor.

In March 1928, he was assigned two years' duty in Nicaragua, serving briefly with the 2d Marine Brigade, and later with the Nicaragua National Guard Detachment. During this period, he organized the Presidential Guard, served as aide and personal bodyguard to President Jose M. Moncada, and commanded a battalion of the Guardia Nacionale in jungle operations against organized bandits in northern Nicaragua.

Upon his return to the United States, he was an instructor at the Basic School, Marine Barracks, Philadelphia Navy Yard; completed the Command and General Staff School, Fort Leavenworth, Kansas; and served as an instructor again at Marine Corps Schools, Quantico. From January 1935 until May 1937, he was stationed in China, serving as a member of the Marine Detachment at the American Embassy in Peiping. In June 1937, he began a three-year assignment as a section chief at Marine Corps Schools, Quantico, then served as Executive Officer of the 5th Marine Regiment at Quantico and Guantanamo Bay, Cuba.

When World War II broke out, he was serving as Chief of Staff, Amphibious Force, Atlantic Fleet (later redesignated Amphibious Corps, Atlantic Fleet). In September 1942, he joined the Amphibious Corps, Pacific Fleet, in San Diego, California, as Chief of Staff, and performed duty in Alaska in July and August 1943 during the planning and training phase of the assault on Attu and Kiska in the Aleutians. Immediately after this, he assumed duty as Chief of Staff of the 5th Amphibious Corps and embarked for the Pacific area. Overseas, he was promoted to brigadier general in November 1943 (with rank from September 1942), and was assigned additional duty as Deputy Commander, 5th Amphibious Corps. For exceptionally meritorious service during the assault and capture of Kwajalein, Saipan, and Tinian, he received two awards of the Legion of Merit, both with combat "V". He also performed additional duties during the Marianas campaign as Chief of Staff of East Marine Force, Pacific.

Following the Marianas operation, he was promoted to major general in September 1944, and the following month assumed command of the 3d Marine Division. He led the 3d Division in the battle for Iwo Jima where members of the division were awarded the Presidential Unit Citation for heroism, and he was awarded the Distinguished Service Medal.

Immediately after the war, as Commanding General of the 3d Marine Division then stationed on Guam, he organized vocational schools on Guam to aid Marines under his command in postwar trades and skills, prior to their return to civilian life.

In October 1945, General Erksine was ordered to Washington and as a result of a special Congressional Act was appointed Administrator of the Retraining and Reemployment Administration (RRA). In June 1947, upon his request to return to duty with the Marine Corps, the general assumed command of the Marine Barracks, Marine Training and Replacement Command, Camp Pendleton, California. The following month, with the return of the 1st Marine Division from China to Camp Pendleton, he became Commanding General of the 1st Marine Division, as well as Commanding General of Camp Pendleton. In May 1949, he was assigned additional duty as Deputy Commander of Fleet Marine Force, Pacific.

During this period, the Marine air units at El Toro were molded together with the 1st Marine Division troops at Camp Pendleton, and it was this combination that embarked for Korea when hostilities erupted in that area.

In June 1950, the Secretary of Defense appointed General Erskine as Chief of Military Group, Joint State-Defense Mutual Defense Assistance Program Survey Mission to Southeast Asia. In carrying out his assigned duties with the Mission, he visited the Philippines, French Indochina, Malaya, Thailand and Indonesia. Upon completing this assignment, General Erskine received orders in December 1950 directing him to assume duties as Commanding General of the Department of the Pacific, San Francisco, California. He also performed additional duties as Member of the Advisory Group, Western Sea Frontier; and Commanding General, Marine Corps Emergency Forces, Western Sea Frontier.

In July 1951, as a lieutenant general, he became Commanding General, Fleet Marine Force, Atlantic. He was authorized to retire from active service by a Special Act of Congress in June 1953 for the purpose of accepting a position as Assistant to the Secretary of Defense as Director of Special Operations of the Department of Defense, and served in this post for over eight years, until 31 October 1961.