text: Armies
  8TH ARMY UNITED STATES FORCES KOREA
  Caution: The U.S. Army currently fields one standing field army
  headquarters, the 8th, which is located in Korea. The 8th U.S. Army is
  optimized for the extremely mature nature of the Korean theater of
  operations, and for the robust alliance structure that has developed over
  the last 70 years. However, the 8th U.S. Army may represent an
  imperfect model for the wide range of activities and responsibilities
  required of a field army operating in another region, under vastly
  different conditions.
  3-122. The Army constitutes a field army when a GCC or theater army commander perceives the
  requirement to command and control beyond the capabilities of a theater army or a corps headquarters. These
  situations occur when multiple corps-sized formations, including U.S. Army and multinational corps, or a
  Marine expeditionary force, operate in a single operational area.
  3-123. When constituted, the field army provides Army, joint, and multinational forces with a headquarters
  capable of performing in a variety of ways across the range of military operations. Field armies are most
  likely to be employed in theaters where peer adversaries have the capability of conducting large-scale combat.
  These regions include U.S. European Command, and USINDOPACOM.
  3-124. However a field army is constituted, the GCC incurs significant risk in theaters with a peer adversary
  without a standing field army, an authorization document to create a field army, or a forward positioned
  corps. The time required to transition responsibilities from the theater army to a field army depends on how
  quickly additional capabilities can reach full operating capability. In OEs where an enemy has significant
  preclusion capabilities, this could be so long that it may not be possible to deny enemy operational and
  strategic objectives without incurring unacceptable costs. The same would likely be true if there were no
  forward positioned corps that could rapidly assume command and control of Army and other land forces in
  an operational area. However, when constituted, field armies provide JFCs and allied organizations with
  additional capability and capacity to counter peer threats. The field army headquarters is the preferred choice
  for serving as a JFLCC.