This is the 9951st most frequent Polish word.
armią
"Armią" means "army" in English.
Here, "armią" is used as an instrumental case form of "armia," referring to the military force involved in meeting someone.
Spotkaliśmy się z armią na granicy.
We met with the army at the border.
In this sentence, "armią" is not used; instead, "armii" is utilized to show the possessive relation between strategy and the army.
Rozmawiają o strategii armii.
They are discussing the army's strategy.
Again, "armią" is in the instrumental case, denoting accompaniment with the mentioned military force during an activity.