When the subject of the sentence is the doer, the verb is in Active Voice. When the subject of the sentence is the receiver of the action, the verb is in Passive Voice.
Here, the subject 'the enemy' performs the action of defeating. If the given sentence is changed into passive voice, then the subject in the active voice becomes object in the passive voice.. Word 'by' is introduced. To convert Present Perfect into passive form, we use has/have + been + 3rd form of the verb. So, correct formation is:
New subject (Our army) + verb (has been defeated) + by + object (our enemy).
