A historic phone call Friday between the presidents of the United States and Iran could mark the end of 34 years of enmity.
Or it could be another missed opportunity.
In the weeks ahead, clear signs will emerge whether a real diplomatic breakthrough is possible. Here are some possible signals that may telegraph whether success or failure lies ahead:
Enrichment in Iran?
Throughout his New York "charm offensive," Iranian President Hassan Rouhani made one demand clear: Tehran will reject any agreement that does not allow it to carry out some uranium enrichment on its soil.
As my longtime friend, and former colleague, Scott Peterson noted in the Christian Science Monitor, Rouhani has made it clear that this is where the Iranians will not budge.
"There's nothing we seek to hide," Rouhani told American media editors, according to Peterson, "40 countries are doing enrichment. We want nothing less, nothing more."
Analysts argue that if Rouhani agreed to no enrichment, it would be political suicide. Rouhani repeatedly stated during his run for president in Iran that the country would maintain a peaceful nuclear program.
"Zero enrichment is not a viable endgame," said Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace.
Israeli officials have long called for an end to all uranium enrichment in Iran. They warn that enriching uranium for energy and medical uses creates an opportunity for illicit enrichment to a level that can be used for nuclear weapons.
In his speech to the United Nations Tuesday, President Barack Obama signaled that he would be willing to compromise on this point. "We respect the right of the Iranian people to access peaceful nuclear energy," Obama said.
For years, Iran has rejected calls for full inspections. So watch for signs of a compromise where Washington accepts limited enrichment inside Iran in exchange for comprehensive inspections.
If not, the talks will quickly unravel.
Obama must win congressional support that the real goal of American policy is to block Tehran from obtaining a nuclear weapon not regime change. Skeptical members of Congress are likely to argue that Iran's regime cannot be trusted under any circumstance. The economic sanctions that have proved so devastating to Iran's economy should continue, they will insist, until Iran's government has collapsed.
Again, Obama is signaling a major change in U.S. policy. "We are not seeking regime change," he said in his speech to the United Nations.
Can Obama convince skeptical members of Congress to adopt his position?
American officials are clearly seeking Iran's cooperation on possibly easing President Bashar al-Assad out of power in Syria. Washington is probably not asking for much - most likely Iranian support for an Assad-less, Allawite-dominated transitional government that will look much like the current regime. As jihadists steadily gain strength in the Syrian opposition, the United States has less interest in the collapse of the Syrian government and army.
But Assad's continued military strength would also suggest that Iran has no incentive to compromise on Syria. Tehran's policy of supplying Assad with weapons and fighters from both Hezbollah and Iran has been working.
If the Iranians refuse to part ways with Assad, the Obama administration may be less willing to compromise on enrichment.
A false start in Geneva?
On October 15, at the P5+1 the five permanent U.N. Security Council members (United States, Russia, Britain, France, China) plus Germany are again set to meet with Iranian officials in Geneva. The fact that Iran quickly agreed to the logistics of this next round of talks was unusual. In past, determining the time and location has itself taken weeks.
But U.S. officials likely expect a detailed proposal from Foreign Minister Javid Zarif regarding what Tehran might accept. If he instead presents only more vague proposals and delaying tactics emerge, expect momentum to slow radically.
For Rouhani, and to a lesser extent, Obama, speed if of the essence. Both leaders face stiff challenges from their right on any deal.
Zarif said in New York that Iran was interested in reaching an agreement and implementing it within one year. A credible agreement that both Obama and Rouhani can hail as progress is needed in months.
If not, both leaders can expect to be accused of capitulation by their critics.
On Monday, Israeli Prime Minister Benjamin Netanyahu will meet with President Obama in Washington. Any shift in Netanyahu's deeply skeptical comments about Rouhani could be a sign that Iran is offering a more substantive proposal in private. Again, all depends on the restrictions Iran would accept on its nuclear program none of which emerged in Rouhani's New York public-relations blitz. Sadjadpour, the Iran expert, said major concessions could win limited Israeli support.
"If Obama can negotiate a deal in which Iran has meaningfully capped its production and stockpile of enriched uranium," Sadjadpour said, "as well as agreed to far greater transparency still a big if -I suspect the Israelis will reluctantly acquiesce."
Watch closely. Those "ifs" will be answered in the days and weeks ahead. Then Rouhani's trip to New York and phone conversation with Obama will emerge as either historic or empty.
(The author is a Reuters columnist. The opinions expressed are his / her own)