Estimates of the recognised Greek Muslim minority, which is mostly located in Thrace, range around 100,000, (about 1% of the population). Some of the Albanian immigrants to Greece come from a nominally Muslim background, although most are secular in orientation. Following the 1919–1922 Greco-Turkish War and the 1923 Treaty of Lausanne, Greece and Turkey agreed to a population transfer based on cultural and religious identity. About 500,000 Muslims from Greece, predominantly those defined as Turks, but also Greek Muslims like the Vallahades of western Macedonia, were exchanged with approximately 1.5 million Greeks from Turkey. However, many refugees who settled in former Ottoman Muslim villages in Central Macedonia, and were defined as Christian Orthodox Caucasus Greeks, arrived from the former Russian Transcaucasus province of Kars Oblast, after it had been retroceded to Turkey prior to the official population exchange.