When did Austria invade Serbia in World War I ?
When Bulgaria attacked both Serbia and Greece on 16 June 1913 , it lost most of its Macedonian region to Serbia and Greece , the Southern Dobruja region to Romania , and Adrianople ( the present - day city of Edirne , which had been conquered by a combined Serbian / Bulgarian force ) to Turkey in the 33 - day Second Balkan War , which further destabilized the region .