With the rise of nationalism in the Ottoman Empire, he joined the Greek army and the Ethniki Etaireia and proceeded to participate in the Greco-Turkish War as a captain.

[4] For this, Gennadis was promoted to major general on October 26, 1912, and took command of the 5th Infantry Division and proceeded to liberate the rest of Western Macedonia.

After the war, he remained with the Division as he was initially stationed at Drama before being transferred to Kavala along with being assigned to the IV Army Corps.

[3][5] When World War I and the National Schism occurred, Gennadis supported Constantine I of Greece which caused him to be dismissed from the army and exiled to Santorini in 1917.

He re-entered service after the monarchists won the November 1920 Greek elections and was made commander of the gendarmerie.