Constantine I (1868–1923), a king of Greece (1913–17 and 1920–22). He was born in Athens, received a military education in Germany, and was married to the sister of Kaiser William II. Constantine was blamed for the defeat of the Greek forces that he led against the Turks in 1897. He regained popularity by leading the Greek army to victory over the Turks in the Balkan Wars (1912–13), which doubled Greek territory. He became king upon the assassination of his father, George I, in 1913.

At the start of World War I Constantine was pro-German, although he proclaimed Greece's neutrality. He was opposed by Premier Eleutherios Venizelos, who set up a provisional government to support the Allies. An Allied ultimatum and a popular vote supporting Venizelos forced Constantine into exile in 1917. His second son, Alexander, became king.

After the death of Alexander and the election defeat of Venizelos, Constantine was recalled by a plebiscite in 1920. A war with Turkey was then in progress, which the Greeks lost, and in 1922 Constantine was again forced to abdicate.