HARI SREENIVASAN: The president met this afternoon at the State Department with members of his national security team to discuss the fight against ISIS and the state of the larger war in Syria, as a key deadline approaches tomorrow for a cessation of hostilities.

He spoke to reporters moments ago.

PRESIDENT BARACK OBAMA: And none of us are under any illusions. We're all aware of the many potential pitfalls. And there are plenty of reasons for skepticism.

But history would judge us harshly if we didn't do our part in at least trying to end this terrible conflict with diplomacy. If implemented — and that's a significant if — the cessation could reduce the violence and get more food and aid to Syrians who are suffering and desperately need it.

It could save lives. Potentially, it could also lead to negotiations on a political settlement to end the civil war, so that everybody can focus their attention on destroying ISIL. And that's why the United States will do everything we can to maximize the chances of success in this cessation of hostilities.

HARI SREENIVASAN: Now, what are the realities for a potential halt to some of the violence and delivery of much-needed aid in Syria?

Chief foreign affairs correspondent Margaret Warner reports.

MARGARET WARNER: There were definitely mixed signals of progress before tomorrow's midnight cease-fire deadline in Syria. Aid trucks rolled into the rebel-held Damascus suburb of Ghouta yesterday, long besieged by the Assad regime.

But in Syria's east, a U.N. airdrop of food went awry. Earlier in the week, Russia claimed it's pared back bombing against U.S.-backed rebels.

MAJ. GEN. IGOR KONASHENKOV, Russian Defense Ministry Spokesman (through interpreter): Russian aviation performs no strikes in these regions where we have received or are continuing to receive claims to cease the fire and start negotiations.

MARGARET WARNER: But, today, there were reports of Russian bombing of rebel-held areas in Syria's northwest and regime bombing in a suburb of Damascus.

The last-ditch agreement, brokered Monday by the U.S. and Russia, called for a — quote — "cessation of hostilities." It exempts operations against ISIS, the al-Qaida linked Jabhat al-Nusra, and terror groups on a U.N. list.

The Syrian government of President Bashar Assad has signed on, as have some rebel groups. But the major Saudi-backed opposition group said it would commit to just two weeks.

SALEM AL MESLET, High Negotiations Committee Spokesman: To be frank with you, we do not trust Russia, we do not trust this regime.

MARGARET WARNER: And U.S. ally Turkey said it wants to continue hitting Syrian Kurds linked to Turkey's Kurdish terrorists.

The day after it was announced, in Washington, senators pressed Secretary of State John Kerry.

WOMAN: I just hope it's not a rope-a-dope deal.

JOHN KERRY, Secretary of State: Well, it may be. I'm not going to vouch for this. I'm not going to say this process is sure to work, because I don't know. But I know that this is the best way to try to end the war, and it's the only alternative before us, if indeed we're going to have a political settlement.

MARGARET WARNER: Many Syria experts are skeptical too. Robert Ford, former ambassador to Syria, resigned in 2014 over the president's refusal to arm the Syrian rebels. Now a fellow with the Middle East Institute in Washington, he also teaches at Yale.

ROBERT FORD, Former U.S. Ambassador to Syria: Secretary Kerry has made huge, huge, huge efforts, but it's too early to say if it represents any progress. The cease-fire is going to be a huge uphill. The Russians and the Syrian regime have incentives to keep fighting. I don't think the war is over by any means.

MARGARET WARNER: But Prem Kumar, who worked on Syria on the National Security Council until last year, says it's an important first step.

PREM KUMAR, Albright Stonebridge Group: It is an important process that will hopefully lead to increased humanitarian assistance, and then, longer term, if the process holds, to discussions about the longer-term political issues that have bedeviled Syria for several years now.

MARGARET WARNER: He thinks it marks a welcome evolution in U.S. thinking, away from regime change.

PREM KUMAR: What is really important about this recent initiative is that I think it begins to shift from one in which the U.S. and its partners were trying to increase pressure on Assad to negotiate his own departure to trying to freeze the conflict, provide humanitarian assistance, and set up a process to address the longer-term political issues.

MARGARET WARNER: What has caused that shift? The sudden insertion of Russian bombers and advisers late last September.

PRESIDENT VLADIMIR PUTIN, Russia (through interpreter): The only right way to fight international terrorism in Syria and on the territory of the states neighboring it is to act preemptively.

MARGARET WARNER: For months, as Secretary Kerry negotiated for a Syria peace process with Russia's foreign minister, Russian bombers were dramatically tilting the battlefield. Five months ago, the rebels were gaining ground.

Now Assad's forces have retaken key areas in the west and surrounded Aleppo, Syria's largest city. And the Islamic State has expanded in the east. Ford sees Russia, along with Iran, Turkey and Saudi Arabia, all backing different fighting elements, as exerting far more leverage now than the U.S.

ROBERT FORD: Russia absolutely is in the driver's seat, and the Americans are watching the car drive by. The Americans are becoming basically irrelevant.

MARGARET WARNER: Former French Foreign Minister Laurent Fabius said the U.S. lost credibility when it didn't follow through on the President Obama's threat to strike Syria for using chemical weapons in 2013. He added, "One doesn't get the sense that there is a very strong commitment. And obviously the Iranians and Russians feel that."

Indeed, Mr. Obama repeatedly resisted calls to arm moderate rebel groups, as he explained to The New York Times in 2014.

PRESIDENT BARACK OBAMA: It has always been a fantasy, this idea that we could provide some light arms or even more sophisticated arms to what was essentially an opposition made up of former doctors, farmers, pharmacists and so forth.

MARGARET WARNER: Mr. Obama's aides say he feared arming inexperienced rebels would draw the U.S. down a slippery slope of deeper military involvement. But Ford says the president's unwillingness did just that.

ROBERT FORD: I think that their inaction put us on a slippery slope. Frankly, we now have American military personnel operating in Syria, Margaret. None of us wanted that in 2011, in 2012, in 2013. But now we're flying combat operations against the Islamic State, not Syrians taking care of it, Americans having to help Syrians take care of it.

MARGARET WARNER: And, in fact, the administration is now asking Congress for additional funds for those operations. So, what happens if this cease-fire fails? On Tuesday, Kerry hinted that deeper military aid could be forthcoming.

JOHN KERRY: There is a significant discussion taking place now about plan B in the event that we don't succeed at the table.

MARGARET WARNER: Foreign Relations Committee Chair Bob Corker was dubious.

SEN. BOB CORKER (R), Tennessee: There is no plan B. Russia knows there will be no plan B.

ROBERT FORD: I saw Secretary Kerry talk about plan B. That's nice. We will see if there's anything there.

MARGARET WARNER: Kumar said there is the possibility of giving the rebels more powerful weaponry or creating safe-zones inside Syria. But even he doubts how much more the president will do in his remaining 11 months.

PREM KUMAR: I think it is unlikely that the administration is going to do a 180-degree turn in its policy on Syria, but I think if the president begins to believe that there is progress in sight, and that the U.S. needs to do more in order to achieve an end to the conflict, then I think he would be willing to consider it.

MARGARET WARNER: A long road ahead that depends heavily on what happens this weekend.

For the "PBS NewsHour," I'm Margaret Warner.