This is the 2857th most frequent Russian word.
армию
"Армию" in English means "army" (accusative case).
The word 'армию' here refers to a military force and is in the accusative case because it is the direct object of the verb 'видел'.
Я видел армию солдат в поле.
I saw an army of soldiers in the field.
In this sentence, 'армию' means a specific group organized as a military force and is again in the accusative case as it is the direct object of the verb 'готовим'.
Мы готовим армию ко вторжению.
We are preparing the army for the invasion.
Here, 'армию' is used metaphorically to describe a large collection or group, and it is in the accusative case as it serves as the direct object of the verb 'принес'.