At the outbreak of World War I in August 1914, the Kingdom of Greece remained neutral. Nonetheless, in October 1914, Greek forces once more occupied Northern Epirus, from where they had retreated after the end of the Balkan Wars. The disagreement between King Constantine, who favoured neutrality, and the pro-Allied Prime Minister Eleftherios Venizelos led to the National Schism, the division of the state between two rival governments. Finally, Greece united and joined the Allies in the summer of 1917.
Greece had emerged victorious from the 1912–1913 Balkan Wars with its territory almost doubled, but found itself in a difficult international situation. The status of the Greek-occupied eastern Aegean islands was left undetermined and the Ottoman Empire continued to claim them, leading to a naval arms race and mass expulsions of ethnic Greeks from Anatolia (Asia Minor). In the north, Bulgaria, defeated in the Second Balkan War, harbored plans for revenge against Greece and Serbia.
Greece and Serbia were bound by a treaty of alliance, signed on 1 June 1913, which promised reciprocal military assistance in case of an attack by a third party, referring to Bulgaria. However, in the spring and summer of 1914, Greece found itself in a confrontation with the Ottoman Empire over the status of the eastern Aegean islands, coupled with a naval race between the two countries and persecutions of the Greeks in Asia Minor. On 11 June, the Greek government issued an official protest to the Porte, threatening a breach of relations and even war if the persecutions were not stopped. On the next day, Greece requested the assistance of Serbia should matters come to a head, but on 16 June, the Serbian government replied that due to the country's exhaustion after the Balkan Wars, and the hostile stance of Albania and Bulgaria, Serbia could not commit to Greece's aid and recommended that war be avoided. On 19 June 1914, the Army Staff Service, under Lt. Colonel Ioannis Metaxas, presented a study it had prepared on possible military options against Turkey. This found that the only truly decisive manoeuvre, a landing of the entire Hellenic Army in Asia Minor, was impossible due to the hostility of Bulgaria. Instead, Metaxas proposed the sudden occupation of the Gallipoli Peninsula without a prior declaration of war, along with the clearing of the Dardanelles and the occupation of Constantinople so as to force the Ottomans to negotiate. However, on the previous day, the Ottoman government had suggested joint talks, and the tension eased enough for Greek Prime Minister Eleftherios Venizelos and the Ottoman Grand Vizier, Said Halim Pasha, to meet in Brussels in July.
The anticipated conflict would emerge from a different quarter however, when on 28 June, the Assassination of Archduke Franz Ferdinand led Austria-Hungary to declare war on Serbia and initiate the outbreak of the First World War a month later.
Between war and neutrality: August 1914 – August 1915
Political considerations: Venizelos and King Constantine
Faced with the prospect of an initially localized Austro-Serbian war, the Greek leadership was unanimous that the country would remain neutral despite the mutual assistance terms of the alliance with Serbia. Greece was prepared to enter the conflict only in the event of a Bulgarian intervention, in which case the entire balance of power in the Balkans would be jeopardized. Furthermore, as it quickly became evident that the conflict would not remain localized but expand to a general European war, any previous considerations by the Balkan countries were upended. This was notably the case for Greece and Romania: both had a stake in maintaining the favourable status quo in the Balkans, but their interests diverged. Thus, once Romania declared its neutrality and refused to undertake any commitments in the event of a Bulgarian attack on Serbia, Greece could not count on Romanian assistance against Bulgaria or the Ottomans, and was, in the view of Venizelos, effectively left diplomatically isolated in the region.
Furthermore, the Greek political leadership was divided in its views on the likely outcome of the war, and hence on the most appropriate Greek policy regarding the combatant coalitions. Prime Minister Venizelos believed that even if Germany and its allies in the Central Powers prevailed in Central Europe, Britain, with its naval might, would prevail at least in the Near East where Greece's interests lay. Venizelos also considered that Greece's two main rivals, Bulgaria and the Ottoman Empire, were likely to join the Central Powers since their interests aligned with those of Germany. The conflict with the Ottomans over the islands of the eastern Aegean, or the pogroms against the Greeks in the Ottoman Empire in particular, were fresh in his mind. Moreover, as the Ottomans were clearly drifting towards the German camp, the opportunity of joint action with the Allied Powers against them should not be missed. While for the moment Venizelos was prepared to remain neutral as the best course of action, his ultimate aim was to enter the war on the side of the Allied Powers should Bulgaria attack Serbia or should the Allies make proposals that would satisfy Greek claims.
King Constantine I on the other hand, backed by Foreign Minister Georgios Streit and the General Staff, were convinced of Germany's eventual triumph and furthermore sympathized with the German militarist political system. As Greece was highly vulnerable to the Allied navies and thus unable to openly side with the Central Powers, Constantine and his supporters argued for firm and "permanent" neutrality. The thinking of Streit, the King's main political advisor on the subject, was influenced by his fear of pan-Slavism (in the first instance Bulgaria, but ultimately represented by Russia) against which Germany supposedly fought, as well as by his belief that the traditional European balance of power would not be upset by the war, leaving little room for territorial gains by Greece in the event of its participation in the conflict. In particular, and in contrast to Venizelos, Streit believed that even if they won, the Allies would respect the territorial integrity of both Austria–Hungary and the Ottoman Empire.
In addition, the King and his military advisors regarded the German army as invincible, while their differences with Venizelos exposed far deeper ideological divergences in Greek society as well: Venizelos represented the middle-class, liberal parliamentary democracy that had emerged after 1909, whereas the King and his supporters represented the traditional elites. Constantine was profoundly impressed by German militarism, Streit was a major proponent of royalist and conservative ideas, while the highly influential Chief of the General Staff Metaxas—who would later as dictator of Greece in 1936–1941 preside over a Fascist-leaning authoritarian regime—was already toying with proto-Fascist ideas.
This disagreement became evident as early as 6 August, when Streit clashed with Venizelos and submitted his resignation. Venizelos refused to accept it so as to avoid a political crisis, while the King also urged Streit to retract it, for fear that his replacement would allow Venizelos to push the government even further towards a pro-Allied course. Thus, when on 25 July the Serbian government requested Greece's aid under the terms of their alliance, Venizelos replied on 2 August that Greece would remain a friendly neutral. The Greek prime minister argued that an important clause in the alliance agreement was rendered impossible: Serbia had undertaken to provide 150,000 troops in the area of Gevgelija to guard against a Bulgarian attack. Furthermore, if Greece sent her army to fight the Austrians along the Danube, this would only incite a Bulgarian attack against both countries, which possessed insufficient forces to oppose it. On the other hand, Venizelos and King Constantine were in agreement when they rejected a German demand on 27 July to join the Central Powers.