By David Rohde

President Barack Obama's speech to the United Nations General Assembly in New York on Tuesday is not expected to generate much excitement. Battered by his uneven handling of Syria, no bold foreign policy initiatives are likely.

Instead, the undisputed diplomatic rock star of the gathering will be Iran's new President Hassan Rouhani. In his first six weeks in office, the cleric has carried out one of the most aggressive charm offensives in the 34-year history of the Islamic Republic. And the Obama administration responded Thursday, saying the president would be open to having a meeting in New York.

If Obama and Rouhani, who will both address the assembly on Tuesday, simply shake hands in public, it will be the seminal event of the gathering's first day.

"More than any words he might say, Rouhani's greatest gesture would be shaking hands with President Obama," Karim Sadjadpour, an expert on Iran at the Carnegie Endowment for International Peace, said in an email this week. "It would be one of the most significant geopolitical handshakes in years."

For both Obama and Rouhani, the stakes are enormous. They face an increasingly chaotic Middle East. The war in Syria is metastasizing into a regional Sunni-Shia clash. Western sanctions have left Iran's economy's in ruins. And a Middle East conflagration could derail a tepid American economic recovery.

Each also faces bitter opposition from domestic conservatives — ready to attack if they sense any weakness in the three-decade cold war between the two nations.

Despite the risks, however, now is the time for Obama and Rouhani to launch the first direct negotiations between Iran and the United States since the 1979 Iranian hostage crisis. From Iran's pursuit of nuclear weapons to the conflict in Syria, the American-Iranian rivalry is helping fuel instability in the region.

For Obama, a bold move on Iran would be out of character. After his lurching response to the August 21 sarin gas attack in Syria, critics are describing Obama's second term as listless. This cautious president who focuses largely on domestic issues would have to roll the dice with Iran, a nation whose leaders have vexed American leaders for decades.

To be fair to Obama — and his predecessors — the onus for talks to begin lies with Rouhani. For years, Tehran has rejected signals from the George W. Bush and the Obama administration that they wanted direct talks.

Still, Obama should make gestures that reward Rouhani for his new genial tone. In the long-term, talks will aid Obama, even if they fail. As he has argued in Syria, exhausting diplomatic alternatives would make it easier to gain American public support if forced to take military action to stop Iran from producing nuclear weapons.

George Perkovich, a proliferation expert at the Carnegie Endowment, said that Obama and Rouhani should hold bilateral talks focused on a comprehensive agreement on Iran's nuclear program. Incremental agreements reached through the P5 plus 1 — the United States, Britain, France, Russia, China and Germany - will, he says, become bogged down.

"We should push for a big comprehensive deal as soon as possible," Perkovich said, "rather than the incremental step-by-step approach. Obama doesn't have the political capital to reduce the sanctions step by step and Rouhani inevitably will lose political capital."

Perkovich argued that Rouhani and his new foreign minister, Mohammad Javad Zarif, are moderates who have won support for talks from Iran's Supreme Leader Ayatollah Ali Khamenei — for now. Perkovich, who has met both of the new leaders, says they have concluded it is not in Iran's strategic interest to develop nuclear weapons.

"They see that it would totally mobilize the Saudis and everyone else against them," he explained, "and ultimately doesn't benefit them."

Skeptics argue that Rouhani, in fact, represents no change in the regime. They dismiss his letter exchanges with Obama and American television interviews as posturing. They also discount the foreign minister's tweeting a Jewish New Year's greeting.

Mark P. Lagon and Mark D. Walllace, two former Bush administration officials warned last month in Foreign Policy that Rouhani's talk of renewed negotiations was a ruse to give Iran more time to develop a nuclear weapon.

"It is imperative that the international community not fall for this trick," they wrote. "No real change will occur under this theocracy. Cosmetic change is not a reason to give the regime economic relief, and the time it needs to finish its nuclear program."

Skepticism is understandable. But given Iranian hardliners' track record of using lethal force to crush uprisings, there is little chance of Khamenei and his Revolutionary Guard backers being toppled. And given the American public's sweeping opposition to U.S. military involvement in the Middle East, there is little chance of a major American military action against Iran.

The best bet is to gamble that Rouhani is what he says — a moderate trying to outflank his country's conservatives. Not rewarding the bold public steps he has taken will undermine Rouhani's fleeting authority in Iran.

If there is a lesson from Afghanistan and Iraq, it is that U.S. military force allows nationalists to blame foreigners for trying to change their nation. Conservatives in Iran will use an American military action to bolster their own standing and discredit moderates.

In the long-term, it is far more effective to have an Iranian moderate battle an Iranian hardliner than an American soldier. In the end, it is Iranians who will discredit their nation's theocracy, not foreigners.