After the Balkan Wars and the First World War Greece occupied the southern part of Macedonia (34.356 km2 or 51% of the ethnic and geographical territory of Macedonia). The majority of the population in this part of Macedonia were Macedonians, while the rest were Turks, Greeks, Jews and Vlachs. The Macedonian language concerning the compactness of the Macedonian population with the historic tradition, had the status of the most usable one and was spoken by 77% of the total number of inhabitants. Greece carried out the policy of denationalization and assimilation of the Macedonians from the Aegean part of Macedonia. Both the Macedonian name and language were forbidden and their usage was the most severely punished. The Macedonians were named as "Bulgarians", "Slavophiles", "Greeks" or "natives".

The Greek government using all possible means (ecclesiastical, educational and governmental propaganda and terror) endeavoured to alter the ethnic character of the Aegean part of Macedonia, carrying out at the same time, the expelling of the Macedonians and the colonization of the non-Macedonian (mostly Greek) population on the estates of the former ones.

According to the Neuilly Treaty (1919) for a "voluntary" exchange of the population between Greece and Bulgaria, 86.000 Macedonians were forced to emigrate in Bulgaria. According to the Lousanne Treaty between Turkey and Greece (1923), 350.000 Moslems, among whom were 40.000 Macedonians-Moslems, were expelled from the Aegean part of Macedonia. On the estates of the expelled Macedonian population (126.000 in total number) in the Aegean Macedonia the Greek government colonized 618.000 persons, mostly of Greek origin. Consequently to this act, this part of Macedonia lost its ethnic character, so the Macedonian language, spoken by 240.000 Macedonians, got the status of minority language. The Macedonians stayed in Kostur (Castoria), Lerin (Florina) and Voden (Edessa) District as a majority. In 1926 Greece passed a law with which the Macedonian toponyms were altered for Greek ones.

In 1925, under the pressure of the League of Nations, Greece printed "Abecedar" ("The Primer") for the Macedonian children. The primer was printed in Macedonian with Latin alphabet. It was immediately captured and destroyed by the authorities.

The Macedonian population from the Aegean part of Macedonia gave resistance to the Greek policy of denationalisation. The Macedonian colloquial language was nourished among the members of the families. Some of the Macedonians were included in the VMRO (United) or in the communist movement so they were consequently apprehended and persecuted. During this period the Macedonian newspapers were also published.