King Constantine I is restored as King of the Hellenes after the death of his son Alexander of Greece and a plebiscite.

Constantine I was King of Greece from 18 March 1913 to 11 June 1917 and again 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. The eldest son of George I of Greece, he succeeded to the throne following his father's assassination in 1913.

The Kingdom of Greece was the Greek nation-state established in 1832 and was the successor state to the First Hellenic Republic. It was internationally recognised by the Treaty of Constantinople, where Greece also secured its full independence from the Ottoman Empire after nearly four centuries. It remained a Kingdom until 1924, when the Second Hellenic Republic was proclaimed, and from the Republic's collapse in 1935 to its dissolution by the Regime of the Colonels in 1973. A referendum following the regime's collapse in 1974 confirmed the effective dissolution of the monarchy and the creation of the Third Hellenic Republic. For much of its existence, the Kingdom's main ideological goal was the Megali Idea, which sought to annex lands with predominately Greek populations.

Alexander was King of Greece from 11 June 1917 until his death on 25 October 1920.

A referendum, plebiscite, or ballot measure is a direct vote by the electorate on a proposal, law, or political issue. A referendum may be either binding or advisory.