Constantine I, King of the Hellenes (2 August 1868 - 11 January 1923), ruled Greece from 1913-1917 and from 1920-1922.
He was the eldest son of George I of Greece and Olga, Queen of Greece.
He was born 2 August 1868 in Athens. He married, on 27 October 1889 in Athens, Princess Sophie of Prussia (1870-1932). They were parents of:
He succeeded to the throne of Greece on 18 March 1913 following his father's assassination.
He left Greece on 11 June 1917 as he had to abdicate because of his pro-German sentiments and was succeeded to the throne by his second son Alexander as acting King. He went into exile to Switzerland. Alexander died 25 October 1920 and following a plebiscite, Constantine returned as king 19 December 1920.
He abdicated the throne on 27 September 1922, and was succeeded by his eldest son, George II. He spend rest of he's life in exile in Italy.
He died in 1923 at Palermo, Sicily.
See abdication speech 1917 : http://www.firstworldwar.com/source/greece_constantineabdication.htm