At the outbreak of the Spanish Civil War, Petrović chose to join the fight on the side of the Spanish Republic and made his way to Spain along with his friend Sreten Dudić under the pseudonym of Fernández García. Upon their arrival, the two pilots were sent on a 26-day training course in Albacete before being sent to the front. After participating in multiple battles, including the Siege of Madrid, Petrović was killed on 12 July 1937, the same day he received his fifth confirmed kill. Shortly after his death, his brother Dobre arrived in Spain and took his place in the squadron.