セルビア救国政府
After the Axis invasion of Yugoslavia in April 1941, Kostić joined the German-appointed Commissioner Government, which was led by Milan Aćimović. The Commissioner Administration was "a simple instrument of the [German] occupation regime", that "lacked any semblance of power". Kostić was the commissioner for transportation from 30 April until 10 July 1941 when he resigned. After that, he refused to take part in the successor puppet government, the Government of National Salvation led by Milan Nedić.