On the other side, Sunni Arab nations in the Gulf - particularly Saudi Arabia and Qatar - as well as the United States have thrown their backing behind the rebellion, trying to stem the influence of their rival Iran.
Given Tehran's entrenched interest in Assad's survival, it is unclear whether Iran's presence at the conference would have helped in convincing him to bend on a transitional government. But backers of Iran's participation say it would at least have brought engagement in these early stages. If the talks do lead to even small breakthroughs - like deals to create humanitarian corridors to besieged rebel-held areas - Iran couldn't stand in their way by arguing it was not involved.
Foreign Minister Walid al-Moallem called Tehran's exclusion a "big mistake," saying that "it is not possible to ignore Iran's important role in bringing stability to the region."
Asked about the subject at a press conference following the talks Wednesday, U.S. Secretary of State John Kerry acknowledged Iran's ability to "make a difference" but reiterated that it has yet to accept the basis for the talks, which is the establishment of a transitional governing body for Syria.
The United States, Saudi Arabia and the opposition say that government cannot include Assad - a stance Damascus, firmly rejects.
"There are plenty of ways that that door can be open in the next weeks and months and my hope is that they will want to join in a constructive solution," Kerry said.
Cortright said that Tehran could eventually come around to a Syrian government without Assad, if it addresses their needs.
"Iran's goal in neighboring Syria is to have a regime that is friendly to its interests and that protects the Alawite community," he said, referring to the Shiite offshoot sect to which Assad belongs. "But this does not mean Iranian officials are wedded to the discredited Assad regime."
He said that by inviting Saudi Arabia and excluding Iran, "the United States is taking sides in a regional ethnic power struggle. This could exacerbate the deepening Sunni-Shiite divide and further undermine security in the region."
But reaching a stage that could lead to Assad not holding power is likely to be very far down the line.
"We have to be very realistic, the conference won't lead to a political settlement or an end to the conflict," said Ayham Kamel, a London-based Mideast analyst for the political risk assessment Eurasia Group. After the failure of the attempt to remove him by force, "now we are in a different world where an Assad ouster is no longer realistic in the near term."
Instead, he said the aim should be to bring democratic reforms in Syria that reduce the grip on power of Assad and his leadership, paving the way eventually for a post-Assad Syria - and that both Iran and Russia are key to that.
"If you needed a final agreement that included real concessions and eventually finds an avenue for Assad to leave - definitely not in the near term - it would require Iranian and Russian support," he said.
Swedish Foreign minister Carl Bildt, who previously served as a top U.N. official on the post-war Balkans, said negotiations should include anyone with a significant role in the conflict.
"You make peace between enemies, you don't have a peace conference to make peace between friends," Bildt said. "So everyone that should have any sort of relevance should be around the table."
