With the beginnings of World War I well prepared by the two Balkan wars of 1912-1913 (briefly discussed in my review of Richard C. Hall's The Balkan Wars 1912-1913: Prelude to the First World War) and with the Great Powers raring to go at each other after decades of planning and amassing weapons, at the beginning of August, 1914, Germany invaded Belgium and Luxembourg on its way to deliver a first and final blow to France (Home Before the Leaves Fall: A New History of the German Invasion of 1914 by Ian Senior) while Austro-Hungary invaded Serbia to put a swift end to the Slavic upstarts' pretensions (The Gardeners of Salonika: The Macedonian Campaign, 1915-1918, by Alan Palmer). None of it went according to plan.
Greece, triumphant in both of the Balkan wars against its Ottoman and Bulgarian arch-enemies (the enmity towards the Bulgarians going back even further than that against the Turks), was torn between standing at the side of the powers who were responsible both for its liberation from the Ottoman Empire in the 1820's and for some of the territorial gains the Greeks had made at the cost of the Turks in the meantime (this faction was led by the on-again, off-again Prime Minister Eleftherios Venizelos) or with the Central Powers (the King of Greece was related to the Kaiser's family and to the British royal family, but he and the Greek high command were strongly bound by admiration for Prussian military values to Berlin) and officially opted for neutrality.
However, the Entente had "legal" rights on the Greek peninsula dating back to treaties made during Greece's war of independence, and French and British divisions soon landed at Salonika (Thessaloniki) on the Macedonian coast won by the Greeks in the wars of 1912-1913. Alan Palmer gives an excellent history of the politics and warfare taking place in southeast Europe after this point in The Gardeners of Salonika, including the formation of a rival government by Venizelos and the subsequent forced abdication of the Greek King. His son came around to Venizelos' point of view. So, finally, Greece came into the war against the Bulgarians, Austro-Hungarians, Turks and Germans,(*) providing a weight of numbers which was important to the final collapse of the Central Powers on the southeastern front.