exercitus

Exercitus, exercitus, m., fourth declension, and the fourth declension is where readers slip. The nominative singular, the genitive singular and the nominative and accusative plural are all exercitus; only the accusative singular exercitum, the ablative exercitu, the genitive plural exercituum and the dative and ablative plural exercitibus give you a handle. Do not read exercitum as if the word were second declension.

It comes from exerceo, "to keep at work, to drill". A Roman army is literally the trained thing, which tells you what Rome believed won its battles. Copiae is the same body seen as manpower and supply; agmen is that body on the march; acies is it drawn up for battle.