From left to right, Russian Foreign Minister Sergei Lavrov, U.S. Secretary of State John Kerry, Saudi Foreign Minister Adel al-Jubeir and Turkish Foreign Minister Feridun Sinirlioglu stand together before a meeting in Vienna last month. (Reuters)

The Obama administration will be forced this weekend to grapple with major decisions on Syria that it has long resisted making but may now be unavoidable if the president’s diplomatic and military strategies there are to succeed.

In a meeting Saturday in Vienna, Secretary of State John F. Kerry will try to build momentum for a Syrian political transition. Allies at the table plan to challenge him to expand the narrow list of U.S.-approved opposition forces fighting against Syrian President Bashar al-Assad and to recognize Islamist groups the administration has shunned as extremist.

On Sunday and Monday, President Obama will face Turkish President Recep Tayyip Erdogan at the Group of 20 economic summit in the Mediterranean resort of Antalya. Erdogan said Wednesday that Syria will be a “major topic” at the summit and that he will push his long-standing demand for the creation of a U.S.-protected Syrian safe zone along the Turkish border.

Russian President Vladi­mir Putin will also attend the G-20 meeting. Russian bombing of opposition forces in support of Assad has fundamentally altered the equation in Syria, and Putin has his own ideas about political transformation, terrorism and air operations there.

The Vienna meeting is the second in as many weeks since Kerry launched a new effort to resolve the Syrian civil war through diplomatic channels. In addition to the humanitarian disaster the conflict has caused, the administration thinks the continuation of the war undercuts its higher priority of defeating the Islamic State in both Syria and Iraq.

What started out as simple protests in Syria has expanded to civil war and now an international crisis.

As he did with successful negotiations over Iran’s nuclear program, Kerry appears to be counting on the prohibitively high price of failure, along with his enthusiasm and determination, to drive the process forward. Before departing for Vienna, he has scheduled a major speech on Syria Thursday.

“It’s a philosophy based on momentum,” said British Foreign Secretary Philip Hammond, one of the participants. “You get people together, you force them to make some forward movement, keeping them at it, keeping their noses to the grindstone, keep them in a locked room.” Kerry, Hammond said, “wants to make some further significant progress this week.”

The plan is for the rapid-fire meetings to continue until success is achieved. “But if he can’t deliver,” a senior administration official acknowledged, “there will maybe be one more after this and it will fizzle. We just don’t know. I’ve seen Kerry pull rabbits out of hats before.” The official was one of several who spoke on the condition of anonymity about internal administration discussions.

Before the first Vienna meeting at the end of October, Kerry and his Russian counterpart, Foreign Minister Sergei Lavrov, agreed that they would set aside the issue that most divided them — whether Assad could be part of a negotiated transition to a new Syrian government. In addition to U.S. allies in the Middle East and Europe, the 19 attendees also included Assad-backer Iran, invited for the first time to participate in international discussions over Syria.

The Syrian opposition and representatives from Assad government were not invited and will not attend the Saturday meeting. The assumption by participants is that if they can reach agreements among themselves, it will eventually be easier to convince the combatants that a deal is viable and to push them toward making compromises that may be necessary.

Instead of starting with Assad, the first meeting concentrated on points of agreement, primarily their desire for a negotiated political settlement, a cease-fire and passage of humanitarian assistance through zones controlled by combatants. In preparation for Saturday, three working groups have been going through proposals for which opposition groups should be eligible to participate in a transition government, which groups should be considered terrorists and ineligible, and how humanitarian access can by arranged.

Separately, U.N. officials working on possibilities for a cease-fire will present proposals to the group.

While there is broad accord over a terrorist list that includes the Islamic State and Jabhat al-Nusra, al-Qaeda’s Syrian affiliate, agreement beyond that has been elusive. Saudi Arabia, Turkey and other U.S. allies in the fight against the Islamic State are demanding that the United States expand its list of viable opposition groups to include Islamist organizations such as Ahrar al-Sham, or Free Men of Syria, and others.

One of the largest and most powerful rebel organizations, Ahrar al-Sham has at times cooperated with Jabhat al-Nusra and has welcomed some of its former members. The administration, as it has with many other locally supported rebel groups, does not consider it part of the “moderate” opposition eligible to participate in transition plans.

Hammond predicted that settling on a definitive list of terrorist organizations “will require deep breaths on several sides, including the U.S. side. The Saudis are never going to sign off on Ahrar al-Sham being categorized as terrorists.”

“If everybody goes into [the Saturday meeting] saying we’re very clear who we won’t deal with and we’re not moving, we won’t get very far,” he said.

While Kerry is seen as open to expanding the list of acceptable organizations, “I don’t know if the White House will sign off on it,” an administration official said.

Any cease-fire would include an exemption for bombing raids against the Islamic State and likely Jabhat al-Nusra, a complication in the case of the latter because its forces in northwest Syria are co-mingled with other opposition groups. While the United States has rarely targeted that part of the country, Russian airstrikes have centered on the area.

“Basically, they want a free pass to keep hitting people,” said the administration official, noting that the Russians might claim they were targeting only Jabhat al-Nusra while continuing to bomb Assad’s opponents.

Whatever optimism Kerry has appears to be based on his belief that Russia is less concerned about Assad than it is fearful that his removal will cause Syria’s military to collapse, eliminating Russia’s sole foothold in the Middle East and opening the door to the Islamic State. U.S. officials from Obama on down have said since the beginning of Russia’s air campaign in Syria in late September that Moscow is making a “mistake” that will make the situation worse.

Assuming Kerry is correct — and that Shiite Iran can also be persuaded to relinquish some of its influence in Syria in favor of a government with a prominent and perhaps dominant role for Syria’s largely Sunni opposition — the question of Assad will soon have to be put on the table.

Both Russia and the United States have proposed timelines of 18 months for a process that includes establishing a transitional governing body, writing a new constitution and holding elections. While Washington wants Assad’s departure early in that process, Russia is expected to insist that if he leaves at all, it will not be until the end.

In the tangled mess of Syria, resolution of the Assad problem leads directly back to the question of who will be eligible to participate in the transition process — due to be discussed at Saturday’s meeting in Vienna. Most opposition leaders, including those backed by the United States, have said they will not participate unless the timing of Assad’s departure is set. Government representatives fearful of their own futures are unlikely to participate in negotiations that begin with assurances of Assad’s departure.

The outcome of the Vienna meeting will weigh heavily on both the tone and substance of the G-20 summit that begins the next day. Erdogan, who spoke by telephone with Obama this week, said Wednesday that his government is prepared to take unspecific “stronger steps” to support a safe zone where Syrian refugees from the fighting, as well as opposition combatants, can be protected from government airstrikes.

He may find growing sympathy for his position among European governments anxious about the rising tide of refugees from the conflict pouring across their own borders.