4 ideas
It was the first internationally sanctioned compulsory population exchange. Its framers, including Fridtjof Nansen's League of Nations refugee office, presented it as a humane way to end massacres, and later negotiators cited it as a precedent for partitions elsewhere.
The exchange sorted people by religious affiliation, not by language or self-identified ethnicity. So Turkish-speaking Orthodox Karamanlides from central Anatolia were sent to Greece as 'Greeks', and Greek-speaking Muslims from Crete and Macedonia were sent to Turkey as 'Turks'. The Ottoman millet category of religious community was converted directly into a national identity, and that conversion retroactively defined what it meant to be Greek or Turkish.
The exchange can be read as a deliberate state-engineering project, not just a humanitarian cleanup after war. By removing minorities, both governments produced the homogeneous populations their nationalist ideologies assumed. Greece settled refugees in Macedonia and Thrace to secure contested borderlands demographically. Turkey's Kemalist state used the departure of Christian merchant classes to build a unified Muslim-Turkish citizenry.
Exchanged people were strangers where they arrived as well as where they left. Anatolian Greeks in Greece were mocked as 'Turkish seed' and resented by locals, and exchanged Muslims in Turkey were treated as outsiders despite a supposedly shared nation. Homogenization by expulsion did not produce belonging. It created a new internal divide between natives and newcomers that lasted for generations.