After the First Balkan War, Grčište came under Serbian control, and Sionidis moved to the village of Matsikovo (modern Evzonoi), which was at the time on the Greco-Serbian border. During the Balkan Wars, King Constantine I of Greece established his headquarters in his house. Sionidis participated in the Battle of Kilkis-Lahanas, where he was injured. Sionidis also participated in the Battle of Skra-di-Legen during World War I, and worked as a secret agent of the Third Army Corps behind enemy lines, in the area of Strumica.