The Treaty of Qasr-i Shirin, signed in 1639, defined the borders between Turkey and Iran. These borders have remained unchanged for centuries, withstanding the rise and fall of empires, wars, and shifts in the regional balance of power. While the geography has not changed, relations between the two countries have often been renewed, sometimes gradually, sometimes dramatically. As Syria enters a new era after Bashar al-Assad leaves power, this historical balance is facing new and serious challenges.