Soon after the Axis invasion of Yugoslavia in April 1941, Pirjevec joined the partisan resistance in the Liberation Front of the Slovenian People, adopting the battle name Ahac, by which he remained known for the rest of his life.
Soon after the Axis invasion of Yugoslavia in April 1941, Pirjevec joined the partisan resistance. He soon emerged as a political commissar in the military units active in the Liberation Front of the Slovenian People, adopting the battle name Ahac, by which he remained known for the rest of his life.