JUDY WOODRUFF: Syria's civil war has ground on for almost five years, but this day saw a diplomatic step that could begin the process of ending a conflict that's killed 250,000 people and displaced millions more.

Chief foreign affairs correspondent Margaret Warner begins our coverage.

MARGARET WARNER: Syrian government forces and rebel groups battled furiously through the weekend. But some of the shooting is supposed to stop this Saturday. Official word came after President Obama spoke to Russian President Vladimir Putin by phone.

White House spokesman Josh Earnest:

JOSH EARNEST, White House Press Secretary: It will require all of the parties who signed onto this document to follow through on the commitments that they have made. The whole world can see in writing what everyone has committed to, and it's time for the signatories to step up and for the bloodshed to come to an end.

MARGARET WARNER: And from Moscow, Putin called it a real chance to stop the violence.

Secretary of State John Kerry had initially announced a provisional agreement yesterday, after long-distance discussions with Russian Foreign Minister Sergei Lavrov. The plan does allow for continuing U.S. and Russian airstrikes against the Islamic State group, Jabhat al-Nusra, and — quote — "other terrorist organizations designated by the U.N. Security Council."

That last phrase will be key, since the Russians regard nearly all groups fighting the Syrian government as terrorists. It's also unclear if the various fighting factions will actually go along with the cease-fire.

Still, U.N. officials expressed cautious optimism.

STEPHANE DUJARRIC, Spokesman, UN Secretary-General: It is a long-awaited signal of hope to the Syrian people that after five years of conflict, there may be an end to their suffering in sight. Much work now lies ahead to ensure the implementation.

MARGARET WARNER: There's also hope that a cease-fire will allow for the quick relaunch of peace talks between the government and opposition groups. But, for now, there is no respite. Islamic State bombings in Homs and a Damascus suburb yesterday killed 130 people.

JUDY WOODRUFF: And Margaret Warner joins me now.

So, Margaret, what did it take to get this deal?

MARGARET WARNER: Well, U.S. officials believe it took two things, one, that Vladimir Putin had decided he got most of what he wanted on the ground. He shored up Assad enough. The Assad military can handle things. They have taken back some territory from both ISIS and from the moderate opposition, and that he doesn't want to get sucked into the quagmire.

The second thing that was epitomized in the phone call today is Putin wanted to be seen here as a partner to the United States in resolving the conflict in Syria. And he wasn't even a player last September before he sent in troops. So it's not bad work in five months that he is now co-chairing this group with President Obama.

JUDY WOODRUFF: How confident, having said that, is the United States that this is actually going to work?

MARGARET WARNER: Well, Judy, I have to say one official said to me today no one's doing high-fives around here. And another one said, after Ukraine, we don't necessarily believe anything.

But the key concession, I'm told, that Putin has made is that he will no longer take Assad's pretext for bombing the moderate opposition. As we said in the setup, what he has been doing is accepting Assad's definition of terrorists, and so even the moderate opposition supported by the U.S. was being pounded by Russian military, and that if you look at the language of the document, it now says you can pound away at ISIS, the al-Nusra Front or other groups designated terrorists by the U.N., and that actually means something.

JUDY WOODRUFF: Now, Margaret, why has this been so hard to get?

MARGARET WARNER: Oh.

Well, Judy, the big problem is, you know, there are so many players, not only on the ground in Syria, but that everyone has a different mentor, a different benefactor, all with different aims. So, for the U.S., number one is getting rid of ISIS. For the Russians, number one was getting — shoring up their client state Syria.

For all the Gulf states, with the Saudis in the lead, number one was getting rid of Assad. And for the Turks, number one was undercutting the Turkish Kurdish fighters whom they see as cousins to their own terrorist Kurds, the PKK. And so when you have a group like that, they have always known that they're never going to get the parties on the ground to agree to this.

It had to come from the big boys, the 17-member group. But when they couldn't agree on what the objective was, that's why it's been so very difficult.

JUDY WOODRUFF: And, as you point out, so many different interests from so many different directions.

Even so, there's been just steady criticism that the administration hasn't done enough, that so many people have been killed, millions displaced from Syria. How does the administration handle that?

MARGARET WARNER: Well, you're right, Judy, not only tremendous criticism, but that the United States was being played by the Russians, that the Russians got themselves in there promising help to defeat ISIS, and instead used it to pound away at Assad's adversaries.

Their answer — well, Secretary Kerry got a little irritated this weekend and said, well, I have not heard any alternatives from anyone else.

Now, there are alternatives, but we will leave that aside. Their answer is we believe the only way to defeat ISIS, which is our number one objective, is end the conflict in Syria. Number two, we work for a president who is not willing to put in any more military muscle. We are now supporting the moderate opposition somewhat, but nothing serious, no surface-to-air missiles to shoot down planes, nothing like that.

And so really talking is the only — getting all the parties together and trying to get to a political resolution was the only way to go. But a senior White House official said to me last week, you know, if this — this is really a turning point. If this doesn't work, I don't know where we go.

JUDY WOODRUFF: Well, we will see where it develops.

MARGARET WARNER: We will see if it goes anywhere.

JUDY WOODRUFF: But, if it does, a huge development.

MARGARET WARNER: Huge, if it does.

JUDY WOODRUFF: Margaret Warner, thank you.