During WWI, the Fifth and Sixth Marine Regiments were combined to form the 4th Marine Expeditionary Brigade. That brigade was in turn attached to the Army's Sencond Infantry Division.
info from:
ARMY LINEAGE SERIES
MANEUVER AND FIREPOWER
THE EVOLUTION OF DIVISIONS AND SEPARATE BRIGADES
by
John B. Wilson
http://www.history.army.mil/books/Lineage/M-F/index.htm
Chapter III: The Test -- World War I
One Regular Army infantry division, the 2d, was organized in France. When the first troops deployed, the U.S. Marine Corps wanted a share of the action, and Secretary Baker agreed that two Marine regiments should serve with the Army. The 5th Marines sailed with the 1st Expeditionary Division, and Pershing assigned them as security detachments and labor troops in France. Shortly thereafter he advised the War Department that the marines did not fit into his organizational plans and recommended that they be converted to Army troops. The marines, however, continued to press for a combat role. Eventually the Departments of War and the Navy agreed that two Regular Army infantry regiments, initially programmed as lines of communication troops, and the two Marine regiments (one serving in France and one from the United States) should form the core of the 2d Division. The adjutant general informed Pershing of the decision, and Brig. Gen. Charles A. Doyen, U.S. Marine Corps, organized the 2d Division on 26 October 1917 at Bourmont, Haute-Marne, France. The division eventually included the 3d Infantry Brigade (the 9th and 23d Infantry and the 3d Machine Gun Battalion), the 4th Marine Brigade (the 5th and 6th Marines and the 6th Machine Gun Battalion [Marines]), the 2d Field Artillery Brigade, and support units