By 9 September 1922, the Turkish army had entered Smyrna, with the Greek authorities having left two days before. Large scale disorder followed, with the Christian population suffering under attacks from soldiers and Turkish inhabitants. The Greek archbishop Chrysostomos had been lynched by a mob which included Turkish soldiers, and on September 13, a fire from the Armenian quarter of the city had engulfed the Christian waterfront of the city, leaving the city devastated. The responsibility for the fire is a controversial issue; some sources blame Turks, and some sources blame Greeks or Armenians. Some 50,000 to 100,000 Greeks and Armenians were killed in the fire and accompanying massacres. According to the population exchange treaty signed by both the Turkish and Greek governments, Greek orthodox citizens of Turkey and Turkish and Greek Muslim citizens residing in Greece were subjected to the population exchange between these two countries. Approximately 1,500,000 Orthodox Christians, being ethnic Greeks and ethnic Turks from Turkey and about 500,000 Turks and Greek Muslims from Greece were uprooted from their homelands. M. Norman Naimark claimed that this treaty was the last part of an ethnic cleansing campaign to create an ethnically pure homeland for the Turks Historian Dinah Shelton similarly wrote that "the Lausanne Treaty completed the forcible transfer of the country's Greeks." A large part of the Greek population was forced to leave their ancestral homelands of Ionia, Pontus and Eastern Thrace between 1914-22. These refugees, as well as Greek Americans with origins in Anatolia, were not allowed to return to their homelands after the signing of the Treaty of Lausanne.
Question: Across how many years was the Greek population forced to leave their ancestral homelands?
Answer: 8