News that Iran’s and Turkey’s governments have reached an accord on Idlib — a Syrian town that is now the focus of American interests — brings relations between two of the largest and most influential states in the Middle East momentarily out of the shadows.

Iran and Turkey’s rivalry goes back a half-millennium. It included eleven wars, and now remains, in the words of the Washington Institute’s Soner Cagaptay, the region’s “oldest power game.”

So what does the recent accord signify, and how will Iran and Turkey’s competition influence the region’s future?

Iranian and Turkish parallels are noteworthy. Both countries have populations of 80 million. (Egypt, the region’s third-largest country, has 96 million.) Both boast ancient civilizations, long imperial histories, past tensions with Russia and a successful avoidance of European colonialism. In modern times, each came under the rule of a ruthless modernizer after World War I, followed more recently by an even more repressive Islamist leader.

The countries’ current leaders — Iran’s Ali Khamenei and Turkey’s Recep Tayyip Erdoğan — enjoy near-absolute power. Further, both feverishly try to hide this reality under a large and noisy apparatus of elections, parliaments, cabinets, laws and NGOs. Both countries aspire to lead the entire Muslim community. In an era of muted anti-Zionism from Arab states, Tehran and Ankara now lead the charge, with the Islamic Republic of Iran loudly denying the Holocaust, and the Republic of Turkey comparing Israelis to Nazis.

In several ways, Iranians lead the Turks, but the latter are catching up. Ayatollah Khomeini came to power in Iran in 1979, and Erdoğan in 2002. Iran has long enjoyed massive oil and gas reserves, but Turkey has recently built an impressive economic base. Tehran deploys forces abroad, dominating four Arab capitals, while Ankara still fights domestic opponents, especially Gülenists and Kurds. Both governments despise the West, but Iran’s is openly hostile to it, while Turkey formally remains in NATO and ostensibly seeks European Union membership.

Khamenei’s thugs capture American sailors on the high seas, while Erdoğan’s take residents hostage. Conspiracy theories, long an Iranian art form, have made huge strides over the past two decades in Turkey, which may now boast the region’s most fantastical speculations.

Both countries became enthusiastic allies of Venezuela’s dictator, Nicolás Maduro. As head of a longer-established dictatorship, Khamenei can allow relative freedom of expression, compared with Erdoğan’s obsessive desire to control speech and thought — including what basketball players in the United States say, or what travelers transiting through Istanbul’s airport think.

The countries’ most major difference concern the attitudes of their subjects. Whereas Khamenei enjoys the support of only about 15 percent of the populace, Erdoğan can count on some 45 percent, affording Erdoğan a legitimacy and confidence that Khamenei can only dream of. In part, this results from longevity under Islamist rule. The disparate support is also explained by a difference in per capita income — which is only US $4,700 and stagnant in Iran, and $10,700 and rising in Turkey.

Regime collapse in Iran is within sight and would diminish Islamism, encouraging Muslims to move toward a more modern and moderate form of their religion. The Turkish government’s greater popularity — and more advanced version of Islamism — gives it greater staying power that makes it the more worrisome long-term opponent. Thus, the Middle East is likely to witness a grand switch, with Iran on course to moderation and Turkey becoming the region’s supreme danger.

Bilateral relations between the two countries flourished during the first years of Erdoğan’s rule (2002-10), when they shared an Islamist worldview and a suspicion of US intentions in Iraq. But relations then soured, primarily because both regimes seek foreign influence, and — as neighbors — they inevitably clash. The civil war in Syria, where Tehran backs Shiite-oriented jihadis and Ankara backs Sunni jihadis, is their biggest, but not only problem. Other matters also aggravate relations, such as supporting opposing sides in Yemen, Turkish installation of a NATO radar system that tracks Iranian activities, and Iranian support for Al Qaeda against Turkey.

Tensions have reached the point that Ali Vaez of the International Crisis Group believes that Tehran and Ankara are “on a collision course.” Left unchecked, he believes that the present dynamics point “toward greater bloodshed, growing instability and greater risks of direct … military confrontation.” More poetically, Cagaptay observes that the Middle East has room for “one shah or sultan, but not a shah and a sultan.”

In this context, the Idlib accord looks flimsy and transient. Tehran and Ankara will probably soon turn against each other — and, with renewed vigor, continue their perpetual rivalry.

Mr. Pipes (DanielPipes.org, @DanielPipes) is president of the Middle East Forum. © 2017 by Daniel Pipes. All rights reserved.

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