To support the advance into Virginia, the Navy Department detailed a battalion of U.S. Marines for temporary field service with Brig. Gen. Irvin McDowell's Union Army. The Marine Commandant, Col. John Harris, expressed misgivings about the inexperience of his available force. Of the 350 Marines then training at the Marine Barracks in Washington, D.C. most were raw recruits with less than three weeks' service.
During the fight for Henry Hill, the Marines supported the batteries of Captains Charles Griffin and James Ricketts. The battalion endured a galling fire from the opposing Confederate artillery, forcing them to ground the protection. Union troops soon fell back in disorder. After regrouping near the Stone House, the Marines participated in two subsequent attempts to recapture the plateau. Although both attacks failed, the conduct of the marines at First Manassas received praise from Union and Confederate soldiers alike.
Source: U.S. Department of the Interior, National Park Service