A field army (or numbered army or simply army) is a military formation in many armed forces, composed of two or more corps and may be subordinate to an army group. Likewise, air armies are equivalent formation within some air forces, and within a navy the comparable notion is that of a fleet. A field army is composed of 300,000 to 600,000 troops.

FM 100–5, Operations of Army Forces in The Field (with included Change No. 1) 17 December 1971: This manual supersedes FM 100–5, 19 February 1962, including all changes. W. C. Westmoreland: INACTIVE: FM 100–5: FM 100–5, Operations of Army Forces in The Field: 6 September 1968: This manual supersedes FM 100–5, 19 February 1962,