"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 ...
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subordinate to an army group. Likewise, air armies are equivalent formation within some air forces. A field army is composed of 100,000 to 300,000 troops.
Specific field armies are usually named or numbered to distinguish them from "army" in the sense of an entire national land military force. In English, the typical style for naming field armies is word numbers, such as "First Army"; whereas corps are usually distinguished by Roman numerals (e.g. I Corps) and subordinate formations with ordinal numbers (e.g. 1st Division). A field army may be given a geographical name in addition to or as an alternative to a numerical name, such as the British Army of the Rhine, Army of the Niemen or Aegean Army (also known as the Fourth Army)." - (en.wikipedia.org 09.03.2020)
