Why did Greece object to the Republic of Macedonia using the name Macedonia?

Greece objected to the Republic of Macedonia using the name "Macedonia" primarily because Greece possesses its own northern region also named Macedonia.1

Greece viewed the use of the name by a state with a majority Slavic population as an attempt to monopolize a name it considered part of its own exclusive national heritage. Specifically, Greece claimed historical continuity with the ancient Macedonians and Alexander the Great, and it feared that the new state’s use of the name implied territorial or cultural claims to that Greek legacy.1

This "name issue" led to long-standing diplomatic tensions, with Greece blocking the country's membership in the European Union and NATO until the dispute was resolved. The conflict was eventually addressed through the 2018 Prespa Agreement, in which the country officially changed its name to the Republic of North Macedonia, and both nations agreed that the terms "Macedonia" and "Macedonian" would refer to distinct histories and cultural heritages.1