The main backers of Syria's warring factions, including for the first time Iran, have gathered for talks billed as a last chance to pull the country back from destruction.
US officials have played down the chances of an early agreement in Vienna on forming a transitional government to pave the way for Syrian leader Bashar al-Assad's exit.
But, if Secretary of State John Kerry even persuades his Saudi and Iranian counterparts to sit down together at Friday's main talks session, he will have made progress.
Kerry met envoys from two of Assad's staunchest allies - Iran's Foreign Minister Mohammad Javad Zarif and Russia's Sergei Lavrov - in their hotels before the negotiations.
"Now it is the right time to bring Iran to the table," he said, playing down criticism that Tehran has won a seat at the talks by supporting Assad's brutal war tactics.
Later, Kerry and Lavrov were joined by foreign ministers Feridun Sinirlioglu of Turkey and Adel al-Jubeir of Saudi Arabia to discuss the agenda of Friday's meeting.
There, this core group will be joined by Iran and about a dozen more parties with an interest in ending a four-year war that has claimed a quarter of a million lives.
But there may be strong disagreements ahead. Jubeir, in an interview with the BBC, made it clear that Riyadh is sticking by its view that Assad must leave office quickly.
"He will go either through a political process or he will be removed by force," he warned. Saudi Arabia has supplied arms to rebel groups battling Assad's regime.
Russia and Iran have made it equally clear that Assad has a right to play a role in an eventual transition towards a mooted transitional unity government and later elections.
"Apparently those who have tried to solve the Syrian crisis reached the conclusion that without Iran's presence there will be no possibility of reaching a rational solution," Zarif said, according to Iranian state news agency IRNA.
According to Russian news agency RIA-Novosti, Lavrov welcomed Zarif's involvement, telling him Iran's presence made for "a group that is truly representative".
The United States and its Arab and Turkish allies have long been at odds with Assad's backers Russia and Iran over if and when the Syrian strongman should step down.
But a refugee exodus and the rise of the Islamic State jihadist group have forced the world to look again at a conflict that shows no sign of slowing.