ISTANBUL — The Middle East is a place of fast-changing fortunes these days. Just ask the Turkish prime minister, Recep Tayyip Erdogan. Not so long ago neo-Ottomanism was the vogue phrase to describe Turkey’s expanding regional influence, pursued under a catchy dogma of “zero problems with neighbors.”Now there are zero neighbors without problems. Syria is first among them. Erdogan’s fulminations over the suppression of the Muslim Brotherhood in Egypt and the treatment of the Sunni opposition in Syria have the air of the unbalanced outbursts of a lonely man whose moderate Islamism has morphed into an immoderate fury.

Perhaps Erdogan, whose indignation is not groundless, would benefit from the counsel of a neighbor, Iran. Its fast change has involved the sudden embrace of “heroic flexibility.”The phrase, used by the supreme leader, Ayatollah Ali Khamenei, in a recent speech to the Islamic Revolutionary Guards Corps, was buttressed at the United Nations by Iranian advocacy of “prudent moderation.” These unlikely words were uttered by President Hassan Rouhani, whose education in that stronghold of Enlightenment values, Scotland, appears to have had an impact.

Or so it would seem. Sanity is in the Iranian air after the loony lurches of the aberrant Mahmoud Ahmadinejad. Rouhani and President Obama almost met in New York, a near miss that by the standards of Iranian-American estrangement over the past 34 years counts as a radical rapprochement. Hope stirs, once again, for a U.S. breakthrough with Iran of Nixon-to-China proportions.

But such is the volatility of the Middle East today, and such the hall of mirrors in Tehran, that extreme caution is warranted. As the Iranians say, “Not everything round is a walnut” — and not every form of “heroic flexibility” is an olive branch. Iran always operates on at least two tracks; to do otherwise would be simplistic. Its Shiite religion permits, in some circumstances, the embroidering of the truth for the protection of the faith, a divinely sanctioned dissimulation. This is a land where straight talk and virtue are not widely seen to overlap.

The two core Iranian tracks today are evident. The first is Rouhani’s outreach, his rejection of extremism, his conciliatory (and contested) comments toward Jews, and his categorical statement that, “Nuclear weapons and other weapons of mass destruction have no place in Iran’s security and defense doctrine.”The second is the heavy involvement of the elite Revolutionary Guard Corps in defense of the Syrian despot, Bashar al-Assad, and the multifaceted ongoing campaign of its Quds Force commander, Qassim Suleimani, who summoned Hezbollah into Syria and fights under the broad, often murderous banner of resistance to America, Israel and the West.

How Khamenei manages these two disparate currents will have a decisive bearing on whether the current U.S.-Iranian blandishments lead anywhere.

The supreme leader has two models from Shiite lore at his disposal — the conciliation of the second Shiite imam, Hassan ibn Ali, who chose peace over war after his father’s assassination in 661, or the heroic sacrifice of his younger brother, Hussein, who fought to the death against impossible odds. Khamenei has called Hassan’s compromise “history’s most glorious exercise of flexibility.” But it is Hussein’s defiant martyrdom that has held greater sway in the 34-year history of the Islamic Republic.

For now, Khamenei’s use of the phrase “heroic flexibility” suggests he will give Rouhani’s conciliation quest a chance — so long, of course, as Suleimani’s Plan B is there as a fallback. As one religious scholar told the Tehran Bureau news organization, it’s “Hello diplomacy, so long martyrdom.”After more than three decades of non-communication, U.S.-Iranian diplomacy is fraught with potential misunderstanding. The terms of a potential nuclear deal are no great mystery. Broadly, it would involve capping Iran’s uranium enrichment at 5 percent, permitting that enrichment only under intense international supervision, closing the underground facility at Fordow and abandoning any plutonium separation plans — in exchange for the progressive lifting of international sanctions.

But the zigzagging nuclear program, pursued over many years without producing a weapon, has always been political above all. It is the foremost expression of the Iranian Revolution’s essence: The rejection of foreign ideology and tutelage after decades of perceived humiliation by the West. Bomb production was secondary because it was always fraught with danger for the survival of the Islamic Republic; and Khamenei, as the “Guardian of the Revolution,” is in a conservative business.

So the core of the coming U.S.-Iran negotiation lies in whether Obama can reassure Iran that a nuclear deal does not equal renewed subjugation or cooption. At the same time Khamenei must reassure America that Suleimani’s restless pursuit of violent “resistance” against the West and Israel will cease. A nuclear agreement will make little sense if it not broad enough to allow eventual U.S.-Iranian cooperation on a range of strategic issues.

Obama and Rouhani agree that this is no longer a “zero-sum” world. Both used the phrase. But as the fate of Erdogan’s “zero problems” foreign policy suggests, good intentions are not enough. Hussein’s martyrdom is still likely to win out over Hassan’s compromise. But this is the last best chance for a game-changing accord. The United States and Iran must seize it.