Constantine I (Κωνσταντίνος Αʹ, Konstantínos I; – 11 January 1923) was King of Greece from 18 March 1913 to 11 June 1917 and from 19 December 1920 to 27 September 1922. He was commander-in-chief of the Hellenic Army during the unsuccessful Greco-Turkish War of 1897 and led the Greek forces during the successful Balkan Wars of 1912–1913, in which Greece expanded to include Thessaloniki, doubling in area and population. He succeeded to the throne of Greece on 18 March 1913, following his father's assassination.
Constantine's disagreement with Prime Minister Eleftherios Venizelos over whether Greece should enter World War I led to the National Schism. Under Allied duress, the country was essentially split between the pro-Venizelos North and the royalist South, ushering in a protracted civil war. He forced Venizelos to resign twice, but in 1917 Constantine left Greece, after threats by the Entente forces to bombard Athens; his second son, Alexander, became king. After Alexander's death, Venizelos' defeat in the 1920 legislative elections, and a plebiscite in favor of his return, Constantine was reinstated. He abdicated the throne for the second and last time in 1922, when Greece lost the Greco-Turkish War of 1919–1922, and this time was succeeded by his eldest son, George II. King Constantine I of Greece died on 11 January 1923 in exile in Sicily, Italy from heart failure and brain haemorrhage.
Constantine was born on 2 August 1868 in Athens. He was the eldest son of King George I and Queen Olga. His birth was met with an immense wave of enthusiasm: the new heir apparent to the throne was the first Greek-born member of the family. As the ceremonial cannon on Lycabettus Hill fired the royal salute, huge crowds gathered outside the Palace shouting what they thought should rightfully be the newborn prince's name: "Constantine". This was both the name of his maternal grandfather, Grand Duke Konstantin Romanov of Russia, and the name of the "King who would reconquer Constantinople", the future "Constantine XII, legitimate successor to the Emperor Constantine XI Palaiologos", according to popular legend.