Yup. During the wars of the Yugoslav dissolution, the Serbs had volunteers from most of the Orthodox Christian countries nearby fighting in their army: Greeks, Romanian, Bulgarians, Russians.
The army under St. Lazar that sacrificed itself on the Field of Kosovo to stop the Muslim advance into Europe was led by the Serbian Tsar, but included Greeks, Bulgars, and Romanians as well. Kosovo is sacred not just to the Serbs, but to all Orthodox Christians.