Constantine was the king of Greece from 1913 to 1917 and from 1920 to 1922. His pro-German, attitude during World War I caused the Western Allies and his Greek opponents to depose him in 1917 and, having lent himself to Greece's disastrous Anatolian policy after his restoration, he again lost his throne in 1922.

The eldest son of King George I of the Hellenes, Constantine received his higher education in Germany and married to Sophia, sister of German emperor William II. The brother-in-law of the William II, he was determined to keep Greece neutral after the outbreak of World War I, whereas Prime Minister Eleuthérios Venizélos backed the Allied cause. The Allied occupation of Salonika (October 1915), Venizélos' formation of a separate pro-Allied government (October 1916), and an Allied demand for his abdication finally forced Constantine to turn power over to his second son, Alexander, on June 12, 1917, without, however, renouncing his titular right. On Alexander's death and Venizélos' fall from power (1920), Constantine was recalled from exile by a plebiscite. He had to pursue Venizélos' anti-Turkish policies, which led to catastrophic war in Anatolia in 1922. A military revolt cost him his throne for the second time, and he abdicated on Sept. 27, 1922, in favour of his eldest son, who became King George II.