Army
Concept · Conquest · Stellaris 4.5.2
Army (conquest) in the Stellaris encyclopedia: Armies are the ground forces assigned to defend Colonies or invade enemy Planets.
Army in the game's words
The encyclopedia entry as the game shows it in its tooltips; underlined words open their own entry or page here.
Armies are the ground forces assigned to defend 
Colonies or invade enemy 
Planets.
An Army led by a 
Commander may gain bonuses or penalties depending on their Leader Traits.
Armies are either assembled in the Army Hub of Starbases or in the Armies tab of Colonies.
Entries that mention Army
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Source: common/game_concepts (concept_army, also concept_armies) and localisation/english, Stellaris Cygnus v4.5.2 (9776).