Is it permissible for me to use the computer I took from an army camp during the Liberation War in Libya, given that I returned another device to an officer there?

What you did, transgressing against public property, is impermissible. Your return of the two devices to the official is a good act. If the official is authorized to permit their gift, or the gift of one of them, to you, then there is no blame upon you for benefiting from it. Otherwise, his permission holds no weight, and you are obligated to return the device he permitted you to keep. As for the device he ordered you to give to the officer, the officer bears responsibility for it if he neglects its safekeeping, and you have fulfilled your obligation by returning it to the official. As for the remaining device with you, if you predominantly believe that the official will be negligent with it, then refer to Fatwa number 59193.