The Zagros mountains, in Western Iran, stretch for nearly 1,000 miles, from the sands of the Persian Gulf northwest along the modern border with Iraq and Turkey, separating the plains of Mesopotamia from the expanse of the Iranian plateau. Deep gorges and jagged peaks surpassing 14,000 feet buffered ancient empires from one another—Babylon in the Fertile Crescent and, to the east, the great metropolises of ancient Persia. They frustrated more than one invader, including Alexander the Great.