World War I: Greece joins the Allied powers.

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.