Greece during World War I At the outbreak of World War I in August 1914 , the Kingdom of Greece remained a neutral nation . Despite this , in October 1914 , Greek forces moved in and occupied the areas of southern Albania that it claimed ( Northern Epirus ) at a time when the new Principality of Albania was in turmoil . At the same time , the Kingdom of Italy occupied Saseno and later that December the port of Valona . By the end of 1915 , Greek troops abandoned their holdings in Albania without a fight to the expanding Italian forces . Road to War . Greece had signed a defense treaty with the Kingdom of Serbia in 1913 obliging Greece to come to Serbia ' s aid if attacked from the Kingdom of Bulgaria . When Bulgaria began mobilization against Serbia , Greek Prime Minister Eleftherios Venizelos believed that based on the treaty , he could get Greece to join the war on the Allied side if the Allies landed 150 , 000 troops in Salonika . Venizelos failed to bring Greece into the war on the Allied side because King Constantine I was the brother - in - law of Kaiser Wilhelm II , then emperor of Germany . Constantine was married to Sophia of Prussia , one of Wilhelm ' s sisters and had also undergone military training in Germany . Thus , the King and the anti - Venizelists ( opponents of the Prime Minister ) were opposed to joining the Allied side and argued that the Serbo - Greek Treaty was void if a great power fought alongside Bulgaria . Venizelos was removed from office by the King on 5 October 1915 , only to return to the political scene in October 1916 . Greece , as a neutral without the means to resist , was obliged to acquiesce in the arrival of a Franco - British ( and later also Russian ) expeditionary force , formed in part by withdrawals from Gallipoli , transforming Salonika into an Allied military base . ( Keegan 253 ) These Allied forces began to arrive on 3 October 1915 . In the early summer of 1917 , the Greeks handed over Fort Rupel to the Bulgarians , believing it a neutral act , though claimed as a betrayal by the Allies . Nonetheless , the Allies still tried to swing the Greeks to their side . From their positions in Greece , Allied forces ( British , French , and Russian , Italian , and Serb troops ) fought the war from Greek territory engaging Bulgarian forces when they invaded Greece in August 1918 . Greece joins the war . In August 1916 , Venizelist officials staged the coup , prompting Venizelos to leave Athens . He returned in October 1916 and set up a rival government in Salonika . Allied efforts to persuade the royal government in Athens to abandon its neutrality and join them failed , and relations irreparably broke down during the " Noemvriana " , when French and Greek troops clashed in the streets of the Greek capital . The Greek Army , mostly loyal to the royal government , was largely disarmed and obliged to retreat to the Peloponnese , while the warships of the Greek navy were impounded and manned by French crews . Still , King Constantine , who as a fellow royal and relative enjoyed the protection of the Russian Tsar , could not be removed until after the February Revolution removed the Russian monarchy from the picture . In June 1917 , King Constantine abdicated from the throne , and his second son , Alexander , took the throne as King . Venizelos assumed control of the entire country , while royalists and other political opponents of Venizelos were exiled to France and Italy . Greece officially declared war against the Central Powers on 30 June 1917 , and would eventually raise ten divisions for the allied effort , alongside the Royal Hellenic Navy . Participation in the war . The Macedonian front stayed mostly stable throughout the war . Bulgaria had occupied Thrace in northern Greece from Allied forces before Greece ' s entry into the war . In May 1918 , Greek forces under French General Adolphe Guillaumat attacked the Bulgarian forces and defeated them at the Battle of Skra - di - Legen on 30 May 1918 . This was the first major involvement of Greek forces in the war . Later in 1918 , the Allied forces upped up their offensive from Greece into occupied Serbia . In September of that year , Allied forces ( French , Greek , Serb , and British troops ) , under the command of French General Franchet d ' Esperey , broke through German , Austro - Hungarian , and Bulgarian forces along the Macedonian front . The offensive into Bulgaria was stopped when on 18 – 19 September 1918 , the British and Greek armies were decisively defeated by the Bulgarians at the Battle of Doiran . The outcome of this battle saved Bulgaria from being occupied . Bulgaria later signed the Armistice of Thessalonica with the Allies in Thessaloniki on 29 September 1918 . By October , the Allies including the Greeks under General Louis Franchet d ' Espérey had taken back all of Serbia and were ready to invade Hungary until the Hungarian authorities offered surrender . After the war . Being on the winning side , Greece acquired the remaining Bulgarian territory on the Aegean Sea in the Treaty of Neuilly - sur - Seine , Eastern Thrace and the Smyrna area in the Treaty of Sèvres . The Greek military suffered an estimated 5 , 000 dead ( Gilbert 541 ) from their nine divisions that participated in the war .