Ka cika wannan jumlar: Sa'ad da Yaƙin Duniya na I ya ɓarke a shekara ta 1914, Petrović ne ke ja-gorar sojojin da ke gadin Babban tashar jirgin ƙasa na Belgrade. A watan Nuwamba da Disamba na shekara ta 1914, a lokacin farmakin na uku na Austro-Hungary a cikin Serbia, Petrović ya kasance jami'in ma'aikaci a hedkwatar Sojoji na Farko wanda "Armijski đeneral" (laftanar janar) Živojin Mišić ya ba da umurni. A lokacin da yake a wannan matsayi, Rundunar Soji ta Farko ta yi yaƙi da
Yaƙin Kolubara, babban nasara ga Serbia. A cikin 1915, Petrović ya kasance mataimakin shugaban ma'aikata a hedkwatar Rundunar Sojojin Sama "Timočka", kuma an daukaka shi zuwa matsayin "potpukovnik" (laftanar Kanar). Tsakanin Disamba 1915 da Fabrairu 1916, bayan doguwar janyewar Sojojin Sarauta ta Serbia ta hanyar Montenegro, ya jagoranci sansanin sojoji a Valona a Albania. A ranar 10 ga Fabrairun 1916, Sojojin Sarauta na Serbia sun fice daga Albania zuwa tsibirin Girka na Corfu, kuma a can ya sake haɗuwa.