Hardly a day goes by that an Iranian official doesn’t threaten Turkey. Take for instance Maj. Gen. Yahya Rahim Safavi’s recent warning to Ankara: “Turkey must radically rethink its policies on Syria, the NATO missile shield and promoting Muslim secularism in the Arab world, or face trouble from its own people and neighbors.”e

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This is no surprise. Turkish-Iranian rivalry goes back centuries, to the Ottoman sultans and the Safavid shahs. It briefly subsided in the 20th century, when Turkey became an inward-looking nation-state, leaving a vacuum in the Middle East. In the past decade, though, Turkey’s economic growth and emergence as a regional giant under the Justice and Development Party, or A.K.P., have revived its standing. From the Syrian uprising to Iraq’s sectarian convulsions to Iran’s push for nuclear power, Ankara is the main challenger to Tehran’s desire to dominate the region.

Following the A.K.P.’s ascent to power in 2002, the Turks were, initially, not interested in competition with the Iranians and relations between Ankara and Tehran seemed quite warm. Both countries defended the Palestinian cause. Ankara did not appear threatened by Iran’s nuclear project. High-level visits between the two governments became routine and trade boomed.

Meanwhile, shared objections to the Iraq War appeared to bind the Turks and the Iranians. Iran even stopped harboring rebels of the Kurdistan Workers’ Party, or P.K.K., which it had encouraged to attack Turkey because of Ankara’s pro-Western stance. After the Iraq War, Tehran began bombing the very P.K.K. camps it had earlier permitted on its territory, winning points with the Turks.

Then came the Arab Spring. The uprising in Syria put Ankara and Tehran at polar opposite ends of the regional and political spectrum. Given its democratic traditions, Turkey supported the revolution and sided with the protesters; authoritarian Iran continued its support for the Assad regime and backed his brutal crackdown on civilians.

The Syrian uprising has become a zero-sum game: Either Bashar al-Assad will win, or the demonstrators will triumph. Likewise, it has become a proxy war between Tehran and Ankara, in which there will be only one winner.

Hence, all is fair game now between Ankara and Tehran. Encouraged by Iran, Assad ignored Turkish advice to reform. Turkey is now supporting, hosting, and reportedly arming the Syrian opposition. Iran’s response has been to strike at Turkey by once again supporting the P.K.K., which has launched dozens of deadly attacks, killing more than 150 Turks since the summer of 2011.

Competition over Syria has also mobilized fault lines in Iraq, where Turkey and Iran have been supporting opposing camps. Since Iraq’s first democratic elections in 2005, Iran has supported the Shiite-backed Dawa party of Nuri Kamal al-Maliki, while Turkey has backed the secular pan-Iraqi movement of Ayad Allawi. Following months of contention after the 2010 elections, Maliki formed a government in Baghdad, scoring a victory for Tehran.

Maliki has cracked down on Ankara-backed factions, issuing an arrest warrant for Tariq al-Hashimi, Iraq’s vice president and leader of the country’s Sunni community. Hashimi has taken refuge in the Kurdish-controlled part of Iraq. The Kurds, who have until recently despised the Sunni Arabs for their persecution of the Kurds under Saddam Hussein, are now making amends. They are also closely aligning with Turkey to balance Iranian influence inside Iraq.

Turkish-Iranian rivalry in the Fertile Crescent has opened up a can of worms: Iranian leaders attack Turkey’s “secular Islam” and threaten to “strike Turkey” should Ankara act on its commitment to support NATO’s missile defense project by placing radars on its territory.

Turkey, anchored in NATO and oriented toward the Middle East, is a greater threat to Iranian interests than the merely pro-Western Turkey of a decade ago. There is a chance that Iran might become even more aggressive: Some analysts suggest that the Iranian Quds Force, the special-operations unit of the Islamic Revolutionary Guard Corps, might be connecting with the P.K.K. in northern Iraq to target both Turkey and the Iraqi Kurds.

Both countries are slowly showing their hands in the region’s oldest power game. In the Middle East, there is room for one shah or one sultan, but not both a shah and a sultan.