US Army Special Warfare: Its Origins, Psychological and Unconventional Warfare, 1941-1952 [open pdf - 13MB]
"In the first half of the twentieth century, American leaders employed US Armed Forces to support American foreign policy in 'conventional warfare' against the organized, uniformed forces of enemy nations. Although the size and nature of the forces varied in two world wars and Korea, in each of these conflicts the US Army performed its role with regularly organized divisions and without the use of nuclear weapons. Whether infantry, mechanized infantry, armored, or airborne, the division was the basic formation of the Army, the key organization by which strength was measured in conventional war. After World War II, political and military leaders began to consider other forms of conflict in which US forces might be engaged. Organization, equipment, and doctrine were reexamined in view of the possibility of nuclear war, but in this process the division remained a fundamental military organization. Simultaneously, however, a few thinkers began to consider the possibility of forces capable of operating at the opposite end of the conflict spectrum from nuclear war, below the level of conventional war-to consider, in short, a capability to conduct guerrilla, or 'unconventional' warfare. Regular divisions were never designed or equipped for unconventional warfare, so special units, training, and doctrine would be necessary for such a task."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/
Paddock, Alfred H., Jr. US Army Special Warfare: Its Origins, Psychological and Unconventional Warfare, 1941-1952. Washington, D.C.: National Defense University Press, 1982