armią

In a sentence

• Spotkaliśmy się z armią na granicy.

  We met with the army at the border.

  Here, "armią" is used as an instrumental case form of "armia," referring to the military force involved in meeting someone.

• Rozmawiają o strategii armii.

  They are discussing the army's strategy.

  In this sentence, "armią" is not used; instead, "armii" is utilized to show the possessive relation between strategy and the army.

• Byliśmy z armią w trakcie misji humanitarnej.

  We were with the army during the humanitarian mission.

  Again, "armią" is in the instrumental case, denoting accompaniment with the mentioned military force during an activity.