Greece had signed a defense treaty with the Kingdom of Serbia in 1913 that obliged Greece to come to Serbia’s aid if it were attacked from the Kingdom of Bulgaria. When Bulgaria began mobilization against Serbia in 1914, the Greek Prime Minister Eleftherios Venizelos believed that he could get Greece to join the war on the side of the Allies if they landed 150,000 troops in Salonika.
Venizelos failed to bring Greece into the war on the Allied side. His explanation that this was because King Constantine I was a “German sympathiser”. The king and the anti-Venizelists (opponents of the prime minister) were opposed to joining the war and argued that the Serbo-Greek Treaty was void if one of the great powers fought alongside Bulgaria. However, British, 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 in 1915. Venizelos was unconstitutionally 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) 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, Russian, Italian, and Serb troops) fought the war from Greek territory, engaging Bulgarian forces when they invaded Greece in August 1916 in the Battle of Struma.
In August 1916, Venizelist officials staged a coup d’état that prompted Venizelos to leave Athens. He returned in October 1916 and set up a rival government in Thessaloniki, the so-called Provisional Government of National Defence. Entente and Venizelist efforts to persuade the “official” royal government in Athens to abandon its neutrality and join them failed, and relations irreparably broke down during the Noemvriana, when Entente and Venizelist troops clashed with royalists in the streets of the Greek capital. The royalist officers of the Greek Army were cashiered, and troops were conscripted to fight under Venizelist officers, as was the case with the Greek Navy.
Still, King Constantine, who enjoyed the protection of the Russian Tsar as a relative and fellow monarch, could not be removed until after the February Revolution in Russia removed the Russian monarchy from the picture. In June 1917, King Constantine abdicated from the throne, and his second son, Alexander, assumed the throne as king (despite the wishes of most Venizelists to declare a Republic).
Venizelos assumed control of the entire country, while royalists and other political opponents of Venizelos were exiled or imprisoned. Greece, by now united under a single government, officially declared war against the Central Powers on 30 June 1917 and would eventually raise ten divisions for the Entente effort, alongside the Royal Hellenic Navy.