In the United States, during the
Vietnam War the term came to stand for a corps-sized organization with other functions and responsibilities. To avoid confusion with the
corps designations used by the
Army of the Republic of Vietnam and to allow for a flexible organization, MACV and General
William Westmoreland developed the "field force" such as
I Field Force and
II Field Force. Unlike an Army corps, which had a size and structure fixed by Army doctrine, the field force could expand as needed and had other functions such as liaison with South Vietnamese and
civil affairs functions and was flexible enough to have many subordinate units assigned to it. ==Police field forces==