The Greeks were targeted from 1915 to 1923, culminating in the Burning of Smyrna, the city with the second highest population of Greeks in the world at the time (after Athens). The entire Greek section of the town, and the Armenian neighborhoods too, were systemically targeted and set alight by the Ottoman troops. Greek casualties are estimated at 10-100k, with 50-500k refugees fleeing the Ottoman Empire for Greece. It is said that so many Greeks jumped into the harbor to escape the flames that ships had to start fighting them off (lest they all pile on a ship and sink it). Of note, naval vessels of the Allied Powers in the harbor stuck to their newfound neutrality and did not save any civilians.

One should note that the Greeks almost certainly retaliated against Turks in their territory, but certainly not to the same degree or with the systematic precision of the Turks. See here.

The two sides had been on opposite sides of WWI (Greeks with the Allies, Turks with the Central powers), and they continued fighting long after the Treaty of Versailles. The Greeks pushed deep into Anatolia, and came damn close to capturing Constantinople (changed to Istanbul after). The Allies, who had been funding them for the duration of WWI, were concerned about upsetting the balance of power and didn't want Russia to have an ally control the Bosphorus Strait. (The Greeks were friendly with the Russians, but the Turks hated the Russians, and therefore limited the influence the Russians had in the Mediterranean by bottling them up in the Black sea). So the Allies just cut funding, and the Greeks got slaughtered and pushed back to the sea. Smyrna was their last outpost and when the Turks retook it, they did some pretty nasty things.