Wikipedia:
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.

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. However, British and Australian and New Zealand ships and troops were allowed to use the island of Lemnos as a base from which their attack on Gallipoli was mounted. Venizelos was removed from office by the King on 5 October 1915, only to return to the political scene in October 1916.
Venizelos invited a joint 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 1916, the Athens government under King Constantine handed over Fort Rupel to the Germans, believing it a neutral act, though claimed as a betrayal by the Venizelists. Nonetheless, the Allies still tried to swing the official Athens government 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 1916.
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



Greek military formation in the World War I Victory Parade in Arc de Triomphe, Paris. July 1919.
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, Italian, 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.

The Greek military suffered an estimated 5,000 dead from their nine divisions that participated in the war.