Which treaty in 1639 established the Ottoman-Persian border?

The treaty that established the Ottoman–Persian border in 1639 was the Treaty of Zuhab. It was signed on 17 May 1639 at Qasr-e Shirin, in present-day western Iran, between the Ottoman Empire and the Safavid Empire.

The agreement ended the Ottoman–Safavid War of 1623–1639, a conflict driven by rival claims over Mesopotamia, the Caucasus, and frontier regions. The treaty confirmed Ottoman control of Baghdad and much of Iraq, while Safavid Iran retained territories including eastern Armenia, eastern Georgia, Dagestan, and Shirvan.

Zuhab did not create a perfectly surveyed modern border. Its territorial descriptions were often vague, so later Ottoman–Iranian agreements had to revisit disputed sections. Even so, it became the foundation for the frontier between the Ottoman Empire and Iran, and later helped shape the borders of modern Turkey, Iraq, and Iran.

A common mix-up is the Peace of Amasya of 1555, an earlier Ottoman–Safavid agreement. The Treaty of Zuhab is also called the Treaty of Qasr-e Shirin.