When World War I broke out in 1914, Petrović was in command of the troops guarding the Belgrade Main railway station. In November and December 1914, during the third Austro-Hungarian offensive into Serbia, Petrović was a staff officer at the headquarters of the First Army commanded by "Armijski đeneral" (lieutenant general) Živojin Mišić. During his time in this position, the First Army fought the Battle of Kolubara, a decisive victory for the Serbs. In 1915, Petrović was an assistant chief of staff on the headquarters of the Infantry Division "Timočka", and was promoted to the rank of "potpukovnik" (lieutenant colonel). Between December 1915 and February 1916, after the Royal Serbian Army's long withdrawal through Montenegro, he commanded an army camp at Valona in Albania. By 10 February 1916, the Royal Serbian Army had evacuated from Albania to the Greek island of Corfu, and there it regrouped.