Treaty of Lausanne
The Treaty of Lausanne was signed in 1923, after Turkey had won the Greco-Turkish War, of 1919-22. It settles the border between modern-day Greece, and modern-day Turkey, as well as other borders of Turkey. Other countries that signed the treaty include Italy, France, Great Britain, Japan, as well as the Kingdom of Yugoslavia.
Note that Greece and Turkey have had disputes, often focused on the question, which country certain (usually uninhabited) islands and rocks in the Aegean Sea belong to. In 1987, and 1996 these disputes almost led to military conflict.