A Field Force in British and Indian Army military parlance is a combined arms land force operating under actual or assumed combat circumstances usually for the length of a specific military campaign. It is used by other nations, but can have a different meaning.
A Field Force would be created from the various units in an area of military operations and be named for the geographical area. Examples are-
In Australia a Field Force comprises the units required to meet operational commitments.
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.
In counter insurgency type campaigns, select and specially trained units of police armed and equipped as light infantry have been designated as police field forces who perform paramilitary type patrols and ambushes whilst retaining their police powers in areas that were highly dangerous.
Examples of these are: