Bulgaria and Greece were bitter antagonists from the 1870s till the 1960s. From that point onwards, relations have improved dramatically - diplomacy, trade, FDI, people-to-people, tourism. It's a Balkan success story remaining off the radar.
The main border crossing between Bulgaria-Greece at Promahonas/Kulata after Bulgaria joined fully Schengen after midnight. A Balkan boundary set in 1913 is no more.