Greece Declares War on Austria-Hungary, Bulgaria, Germany and Turkey

As a result of the Balkan Wars, Greece successfully increased the extent of her territory and population, a challenging context both socially and economically.

In the following years, the struggle between King Constantine I and charismatic Prime Minister Eleftherios Venizelos over the country's foreign policy on the eve of World War I dominated the country's political scene, and divided the country into two opposed groups.

In the aftermath of WWI, Greece fought against Turkish nationalists led by Mustafa Kemal, a war which resulted in a massive population exchange between the two countries under the Treaty of Lausanne. According to various sources, several hundred thousand Pontic Greeks died during this period. Instability and successive coups d'état marked the following era, which was overshadowed by the massive task of incorporating 1.5 million Greek refugees from Asia Minor into Greek society. The Greek population in Istanbul had shrunk from 300,000 at the turn of the century to around 3,000 in the city today. On 28 October 1940 Fascist Italy demanded the surrender of Greece, but Greek dictator Ioannis Metaxas refused and in the following Greco-Italian War, Greece repelled Italian forces into Albania, giving the Allies their first victory over Axis forces on land. The country would eventually fall to urgently dispatched German forces during the Battle of Greece. The German occupiers nevertheless met serious challenges from the Greek Resistance. Over 100,000 civilians died from starvation during the winter of 1941–42. In 1943 virtually the entire Jewish population was deported to Nazi extermination camps.

After Greece declared war on the Central Powers in July 1917, ten divisions of the Greek army fought with great valor along the Macedonian front. In 1918 they routed German and Bulgarian positions and pushed the front line northward. Germany and its allies soon capitulated, and Greek troops were among those who marched triumphantly into Constantinople. At the cost of splitting the nation, Venizelos had brought Greece into the war on the victorious side. To justify the cost of this result and heal the wounds caused by the National Schism, he returned to the Megali Idea.