"The Army announced in the 2006 Army Posture Statement that it had to operationalize the Army National Guard. The decision to operationalize the National Guard was necessary because the Army did not have enough active units to meet requirements for the War on Terrorism. Operationalization of the Army National Guard was a departure from the Cold War force structure. It was not, however, a revolutionary change for the nation's National Guard forces. The research initially focused on determining what was meant by the term 'strategic reserve.' That research revealed that numerous military commanders, both active and reserve, have used the term in reference to U.S. Army force structure. However, military policy documents and statutes do not define that term. The search for a clear definition of strategic reserve and its meaning for U.S. National Guard structure led to the discovery that the National Guard had only recently been constituted as a strategic reserve."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/