Since the succession of Macedonia, the large albanian population in the country have been oppressed and prevented from displaying their nationality in the country. With this said, the conflict began as far back as Alexander the Great, when Albania helped Greece take over Macedonia in supplement to ending the ancient Macedonian language.
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The repression of the Albanian language and Albanian flag by the Macedonian majority in government. Also by the large number of refugees coming into Macedonia from Albania (ie. weak economy)
SOURCES:
"The Macedonian Question: Reform or Rebellion". International Crisis Group (ICG). 2001
"Between Preventive Diplomacy and Conflict Resolution: The Macedonian Perspective on the Kosovo Crisis" by Prof. Biljana Cvetkovska. International Studies Association 40th Annual Convention. Washington, D.C. 1999
https://www.aljazeera.com/programmes/peopleandpower/2015/03/macedonia-facade-150318135440964.html
http://www.businessinsider.com/a-bloody-gun-fight-between-cops-and-ethnic-albanian-terrorists-is-deepening-a-crisis-in-macedonia-2015-5