Turkey and Iran had only basic relations in the first half of the twentieth century. In the 1950s, the two countries closed ranks. Together with Britain, Iraq, and Pakistan, they became allies under the Baghdad Pact in 1955. But Turkey and Iran failed to become close allies. This paper explains that failure by analyzing Turkish and Iranian leaders’ diverging geostrategic priorities, mutual distrust, and domestic troubles from the 1960s onward. In the mid-1970s, Ankara and Tehran wanted to cooperate in the field of arms production and highway construction. But the deterioration of political order in the two countries in the second half of the 1970s meant that their leaders could not realize the joint projects. Turmoil also made Turkish and Iranian leaders suspicious. The Shah worried that political violence in Turkey and Ankara’s deteriorating relations with Washington in the aftermath of the Cyprus crisis of 1974 could endanger Iran. Opinion-makers and political leaders in Turkey had their doubts about Iran: when the Turkish press realized how the Shah had supported the Kurdish insurgency in northern Iraq, they wondered if the Iranian king supported armed Kurdish groups in Turkey as well. Meanwhile, the Turkish political elite, with their military suffering under the U.S. embargo, watched the Shah’s massive arms buildup with a mix of unease and jealousy. The Islamic Revolution and the Iran-Iraq War turned the tables. Iran’s mounting domestic and international problems – revolutionary excess, economic downturn, the hostage crisis, diplomatic isolation – allowed Turkey to deal with its eastern neighbor from a position of strength. As Turkey’s September 1980 coup brought turmoil to an end, it took advantage of Iran’s cold war with the United States and devastating war with Iraq by expanding trade with Iran and Iraq. Beyond the two countries’ Cold War relations, this paper also explores the underlying dynamics that constrain Turkish-Iranian cooperation today. The political systems of Turkey and Iran (the former viewed as secular, democratic, and pro-Western and the latter perceived as religious, undemocratic, and anti-Western) are frequently used to explain their differences. But this paper argues the opposite: the two countries’ desire to become the regional hegemon, as well as the chasm between their preaching peace and democracy without fully practicing them, are the most important reasons for their inability to cooperate. That chasm could make both the Turkish and Iranian “models” irrelevant to the countries undergoing the Arab Spring.