In January 1918, he sailed for France, and as a platoon leader in the 6th Marine Regiment, participated in the Aisne-Marne Defensive (Battle of Chateau-Thierry), where he was wounded in action; also fighting at Belleau Wood; Bouresches; and Soissons.

In the St. Mihiel Offensive, he was so seriously wounded that he was evacuated to the United States in October 1918, spending a year in the hospital and undergoing nine surgeries.

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.

Overseas, he was promoted to brigadier general in November 1943 (with rank from September 1942), and was assigned additional duty as deputy commander, V 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".

Upon completing this assignment, Erskine received orders in December 1950 directing him to assume duties as commanding general of the Department of the Pacific, San Francisco, California.

Upon his retirement from active duty in the Marine Corps, Erskine was advanced to four-star rank, July 1, 1953, by reason of having been specially commended for heroism in combat.

He was authorized to retire from active service by a Special Act of the United States 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 United States Department of Defense, and served in this post for over eight years, until October 31, 1961.