The 1923 Exchange of Populations between Greece and Turkey refers to the first large scale population exchange, or agreed mutual expulsion in the 20th century. It involved some two million persons, most forcibly made refugees and de jure denaturalized from homelands of centuries or millennia, in a treaty promoted and overseen by the international community as part of the Treaty of Lausanne. The document about Exchange of Greek and Turkish Populations was signed at Lausanne, Switzerland, January 30, 1923, between the governments of Greece and Turkey. The exchange took place between Turkish nationals of the Greek Orthodox religion established in Turkish territory, and of Greek nationals of the Muslim religion established in Greek territory.

Many huge refugee displacements and movements occurred in the upheaval following the dissolution of the Ottoman Empire and its evolution into modern Turkey, especially following the Balkan Wars, World War One, and the Greco Turkish War of 1919-1922, which was part of the Turkish War for Independence. These included smaller exchages of Greeks and Slavs, and Turks and Bulgarians.

Following these models, as part of the Treaty of Lausanne almost all Greeks(including Turkish speaking Christian population in middle Anatolia), about 1.5 million, from Turkish Anatolia and Turkish Thrace were expelled or formally denaturalized, and about 500,000 almost all Turks (including Muslim population from Crete speaking a dialect based on Greek language with additional Turkish words, and Muslim Gypsies) were expelled from Greece. The Greeks of Istanbul, Gökçeada (Imbros in Greek) and Bozcaada (Tenedos in Greek), as well as the Turks and other Muslims of Western (Greek) Thrace were exempted from this transfer.

In Greece this was called the "Asia Minor Catastrophe" as it involved the expulsion of about one third of the Greek population from millennia old homelands.

While the populations which were expelled suffered greatly, both the nation states of Greece and Turkey, as well as the international community, saw the resulting ethnic homogenization of their respective states as positive and stabilizing.

References

The section of the Treaty of Lausanne that orders Exchange of Greek and Turkish Populations.

Red Cross Report on the Greek-Turkish Conflict