Promoted to Captain, he fought in the 1941 Invasion of Yugoslavia, after which he joined guerilla forces of General Draza Mihailovic, the Chetniks.

He fled on 13 January 1944 and soon made contact with local Home Army unit, which took him to a hideout in the village of Zubrza near Lwow.

After identity check, Sotirovic was in late March 1944 sent to the Home Army’s 14th Regiment of Jazlowiec Uhlans, commanded by Colonel Andrzej Choloniewski.

Hiding his real identity and presenting himself as French officer Jacques Roman, who had been kept in a German POW camp at Odessa, Sotirovic was taken to a hospital in Rzeszow, where he was freed by members of the NIE organization.

After several clashes with the Ukrainians, Sotirovic, who had been promoted to Major, convinced the warring sides to sign a local truce, on 29 May 1945 in Siedliska.