VIENNA (AP) — The success or failure of the Syrian peace talks this week is tied to the fate of one man who’s not even at the table: President Bashar Assad.

Neither Assad nor any of his representatives are in Vienna, where U.S. Secretary of State John Kerry and senior diplomats from almost 20 other countries began negotiations Thursday. But his future is at the heart of all the talks.

The broad group of governments — an unprecedented collection of countries on the Syria issue — is looking for a plan that might convince Syria’s government and its Western-backed rebels to agree to a national cease-fire and start a long process of “political transition.”

Even bitter foes Iran and Saudi Arabia will be sitting at one table along with their powerful partners, Russia and the United States. That reflects the urgency all parties say they’re attaching to finding a peace formula.

It’s mostly about Assad.

Since the start of Syria’s unrest four years ago, his future has been a stumbling block to all efforts aimed at ending the fighting. President Barack Obama demanded that Assad leave power in 2011. Russia resisted the push by blocking attempts at the United Nations to pressure the Syrian leader and insisting that any new government only be established by mutual consent of both the government and the opposition. That essentially gave Assad veto power over his own would-be replacements.

The United States and its Arab and European allies have since tempered their calls, suggesting that Assad can remain in office for months as part of the transition if he agrees to resign at the end of the process. But even that softened demand has been too much for Russia and Iran, who are both engaged militarily alongside Assad’s forces, fighting rebels supported by the Saudis and the Americans. And neither of the Syrian sides has suggested they would support such a plan.

In Vienna, Kerry is being joined by Russian Foreign Minister Sergey Lavrov and counterparts from other nations, all hoping to get past disagreements that have held back international mediation efforts over the conflict’s four-and-a-half years. More than 250,000 people have been killed and more than 11 million people — half the country’s population — have been uprooted from their homes. The war has led to the rise of the Islamic State and sparked a refugee crisis that has deeply unnerved much of Europe.

“We’ve a lot of very important meetings. We’ll see what happens,” Kerry said as he kicked off his first session, with Austrian Foreign Minister Sebastian Kurz. Kerry then met with the U.N. envoy for Syria, Staffan de Mistura, and Iranian Foreign Minister Mohammad Javad Zarif, his key counterpart in a seven-nation nuclear deal wrapped up in July.

A four-way meeting including Kerry and the foreign ministers of Russia, Saudi Arabia and Turkey was scheduled for the evening, before bigger talks on Friday.

Saudi Foreign Minister Adel al-Jubeir said Wednesday that Assad must step down “within a specific timeframe,” though he didn’t elaborate. That unleashed bitter recriminations from Syrian Information Minister Omran al-Zoubi, who called al-Jubeir a “tumor.”

Even if the countries do reach an agreement on Assad it won’t solve everything. Syrians of all stripes will have to figure out how to stop the violence between the army and the many different militant groups and how to share power in a government comprised of such fierce enemies.

They need a new constitution. They must figure out what to do about groups such as the Nusra Front, an al-Qaida-linked militia that has at times worked alongside Western-backed fighters. They must decide whether to hold people accountable for crimes committed by all sides. And they have to find a way to cooperate to help defeat the Islamic State.

None of those decisions seem reachable without first determining Assad’s future. That includes what “transition” would mean for him, what powers he could maintain as part of that transition, how long the process should take and whether he can compete in a future presidential election, something his international backers refuse to rule out.

“Assad’s role is a key factor here,” State Department spokesman John Kirby told reporters earlier this week, describing a situation that has barely budged in three years.

But Iran’s participation this week is new, representing a realization by the U.S. and Saudi Arabia that peace is unlikely without support from Assad’s key backers.

Tehran’s attendance appeared to bolster diplomatic hopes, with several European and Arab countries making late requests to attend, even as it outraged Syria’s rebels. Sunni-dominated Saudi Arabia has been more outspoken than the U.S. in labeling the talks as a test of Shiite Iran’s seriousness to seek peace.

Iran’s attendance without a shift in position would mean little, and Zarif said his presence should not indicate he was accepting any conditions. Any plan, he told Iran’s official news agency IRNA, should end the fighting, bring together counterterrorism efforts, create a unity government and achieve “a permanent solution accepted by Syrian people and groups.”

Up to now, its leaders have largely echoed Assad’s contention there is no reason for him to go after he won re-election last year in a vote that Western countries called a sham. Assad’s term ends in 2021 and Russian lawmakers who met him recently said he was willing to hold early presidential elections — but would be on the ballot.

Assad himself appears to be digging in. His office issued a statement this week reiterating that he would not consider any political initiatives “until after eradicating terrorism.”