Initially, the exclusion made sense. The United States, which does not have formal diplomatic relations with Iran, was mainly preoccupied — as were other major Western powers — with reaching an agreement with Iran on nuclear weapons. With that deal done, there is every reason to engage Iran on Syria, especially since Russia’s recent military intervention on the side of Mr. Assad has vastly complicated the conflict and added new urgency to the need for a political solution.

Not everyone will be happy to see Iran at the table. Saudi Arabia, deeply suspicious of Iran’s regional ambitions, argues that negotiating with Iran only legitimizes a government that is exerting malign influence in Iraq, Lebanon, Yemen, Bahrain and Syria. The Saudis, who are Sunnis, fought especially hard to keep Shiite Iran, its main regional rival, from the Vienna meeting and it took a trip to Riyadh on Saturday by Secretary of State John Kerry and a phone call on Tuesday by President Obama to Saudi King Salman to win them over.

The Saudis want Mr. Assad removed from power and the Iranians removed from any post-Assad Syria. They have reportedly tried to enlist President Vladimir Putin of Russia in these objectives with offers of assistance for Russia’s battered economy. Mr. Putin reportedly told the Saudis that he agreed that Iran’s role in Syria is too big, but that he could not guarantee Mr. Assad’s removal.

The Saudis aside, most of the negotiators in Vienna would be delighted by progress on a framework for a transition from Mr. Assad to a more inclusive government. While Mr. Assad needs to leave, the change must be done carefully, retaining the institutions that help keep the state intact. Ideally, a political solution would lead to a more unified multinational campaign against the Islamic State and an eventual end to all foreign involvement in Syria.

In an interview in December 2013, Iran’s foreign minister, Mohammad Javad Zarif, said that Iran understood that there could be no military solution in Syria and would not stand in the way of a political solution. Since then however, Iran has doubled down on its military involvement in Syria, while its supreme leader, Ayatollah Ali Khamenei, has said that there will be no cooperation with the United States on issues other than the nuclear agreement.