History.
Originally the greek byzantine empire owned all of modern Greece and Turkey. Then in medieval times turkish steppe people invaded and formed the ottoman empire over both (and the whole balkan and northern africa). Greece then slowly gained independance from them in the 19th century.
The ottoman empire collapsed after WW1 and the turkish republic formed in it’s core territory. But both sides basically claimed that the entire region should be owned by them. Especially Istanbul, wich was the greek capital Constantinople until 1453.
This led to a bunch of territorial conflicts, and anger over past atrocities and religious oppression.
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