JUDY WOODRUFF: We return now to the fight against the Islamic State group and how those efforts might be hindered by Turkey's downing of a Russian warplane.

I'm joined by Nicholas Burns, a career diplomat and former U.S. ambassador to NATO. He's now a professor at Harvard University. And Angela Stent, author of "The Limits of Partnership: U.S.-Russian Relations in the 21st Century." She's a senior fellow at the Transatlantic Academy.

And we welcome you both.

So, just based on what we know, Angela Stent, who do we think was at fault here? Was it the Turks for allegedly violating Russian airspace or was it the Russians for — I'm sorry — the Russians for going into Turkish airspace, or the other way around?

ANGELA STENT, Author, "The Limits of Partnership": Well, apparently, the Russians were only in Turkish airspace for less than a minute.

But this isn't the first time apparently that Russian planes have violated Turkish airspace. The Turks claim that they gave the Russians 10 warnings. The Russians claim that that's not true. But it does appear that they were briefly in Turkish airspace. The question is, could this have been de-escalated? Could the Turks maybe have offered to escort them out of Turkish airspace?

JUDY WOODRUFF: Well, Nicholas Burns, if they were in Turkish — over Turkish airspace, even for less than half-a-minute, was that something that warranted being shot down by the Turkish military?

NICHOLAS BURNS, Harvard University: Well, the history and circumstances are important here.

They did violate Turkish airspace. And, as President Obama said, every country has a right, Turkey has a right to defend that airspace. But the Russians have violated Turkish airspace on several occasions over the last two months. Russian drones have gone across the border.

And the Turks have warned the Russians both publicly and privately. The Russians have also been bombing Syrian Turkmen, ethnic Turkmen villages very close to that border, and the Turks warned the Russians about that. It seems to be the last straw for the Turks.

There are some people who are saying the Turks should have acted differently, as Angela said, that they might have escorted the fighters out, but there was fair warning to the Russians. And as a NATO ally, it's very important that the United States defend this right that every country's border are sacrosanct.

And what the Russians did is clearly illegal under international law.

JUDY WOODRUFF: So, Angela Stent, if the Russians were repeatedly invading Turkish airspace, even for a brief time, the Turks then thought ahead about what they were doing. And what did they accomplish by having done this?

ANGELA STENT: Well, I think the Turks, they disagree with the Russians on what they're doing in Syria. The Turks of course are taking all these refugees from President Assad. The Russians are supporting Assad.

And as Nick said, they have been bombing and killing Turkmen groups in Syria that are protected really by Turkey. So I think this is something that's been brewing for some time. And I think, on a broader point, ever since the Russians began the bombing campaign in Syria and gave the U.S., for instance, one hour's notice to get out of the sky, there has been this brinkmanship and there has been the danger that something like this would happen.

And so I'm sure that the Turks in some senses were just waiting for something like this to happen and felt that they had to take a stance on this.

JUDY WOODRUFF: Nicholas Burns, if that's the case, if the Turks thought in advance and in essence planned to do this the next time a Russian plane crossed their border, what does that say about what the Turks are — what their posture is at this point?

NICHOLAS BURNS: Well, I think President Obama got the balance right today.

He clearly backed the Turks on the legal issue of protection of borders, but he also said that he would spend all of his time trying now to de-escalate this conflict. And I think what that means, Judy, is that the United States is going to be very active both in Moscow and Ankara to counsel both to stand down from any further altercation.

What really needs to happen — and Angela is absolutely correct about this — is, it's a crowded airspace in a very small country. You have American, French, sometimes Arab, Russian aircraft and Turkish aircraft patrolling in a very small geographical area. And so deconflicting those air operations, having channels among the militaries to let each other know when they're going to be conducting combat operations is critical.

The U.S. and Russia have begun to do that under Secretary Ash Carter's leadership. The Turks and Russians clearly have not. And I think it's the role of the United States to try to promote that kind of transparency, because we don't want to see a further incident like. Very dangerous. We haven't had anything like this in nearly 60 years in the NATO relationship with both the Soviet Union and Russia.

JUDY WOODRUFF: Well, before I ask you both about how it gets calmed down, let me just ask you the question the other way, Angela Stent. Why are the Russians repeatedly crossing the border into Turkey? Do they not see that as a provocative offense?

ANGELA STENT: Well, I think the Russians, they are stating that they're back in Syria, that they want to be a leader, if you like, in this coalition and that the Turks don't have the right to do this to them.

And they really want to be — they want the U.S. to come to them and they want this broad coalition. They want to be the leader. And so they're really being — I mean, there is some reckless behavior here, obviously, even though they have been deconflicting, as Nick said, with the U.S. on a bilateral basis, and, by the way, the Israelis, too.

But they're rather selective in the countries with which they have so far being willing to deconflict.

JUDY WOODRUFF: What does this say — to both of you, Nick, to you first. What does this say about the efforts? And you have just mentioned them going along to try to put a coalition together to find some resolution in Syria.

NICHOLAS BURNS: Well, it's tragically ironic that the two countries that France and the United States need to join this coalition against the Islamic State are Turkey and Russia.

Turkey, as you know, has been — has bombed in the recent weeks the Syrian Kurdish groups that have been the most effective fighting force against the Islamic State. The Russians have made a great rhetorical show of saying they're against the Islamic State, but 95 percent of airstrikes are not against the Islamic State, but against some of the Syrian groups, the Sunni and Turkmen groups opposing President Assad.

So, the challenge here is to make the coalition bigger and stronger, to find a way to bring the Russians and Turks in, but also, Judy, to bring the Europeans in. Most of the Europeans are missing in action. Britain is not involved in the air campaign in Syria. And the Arab states that made a big show of joining this coalition several months ago are mainly focused on Yemen, not here.

And so I think that's the challenge for President Obama and President Hollande as they met at the White House this morning.

JUDY WOODRUFF: So, Angela Stent, do you see any prospects for putting this coalition together, getting it stronger, getting it broader, when, as Nicholas has just said, the Russians are going after the anti-Assad rebels? They're not primarily going after ISIS.

ANGELA STENT: I think it's going to be very difficult. We fundamentally — we disagree with the Russians on the fate of President Assad, and then we disagree who the enemy is.

We can agree that it's Islamic State, but, as has been said, the Russians have been bombing mainly groups that are not part of the Islamic State. I think it's going to be extremely difficult. And it would take a resolution on Russia's part to understand that they have to be willing to make compromises on the issue of, you know, which groups you, in fact, are targeting. And so far, we haven't seen very much willingness on their part to do that.

JUDY WOODRUFF: Nicholas Burns, is — do you sense, though, that there are still efforts to bring the Russians around? I mean, I keep reading reports that the administration continues to work on that, but we just haven't seen any sign of it yet.

NICHOLAS BURNS: Well, we need to work with the Russians, mainly because they need the Russians at the negotiating table in Vienna. And Secretary Kerry, I think, clearly understands there won't be any progress on these negotiations — and these are going to be tough, complex negotiations — unless the Russians and Iran and Hezbollah are there.

That's another problem for the Russians. They have a mainly Sunni Muslim population in their own North Caucasus region, but the Russians have aligned themselves with the Shia powers, with Iran, with Hezbollah, and with the Alawite regime in Syria.

And I think if the Russians don't restrain the Syrian government from firing barrel bombs into civilian neighborhoods, the United States ought to consider a no-flight zone with Turkey and other countries to shut down the Syrian air force. That's what Secretary — former Secretary Hillary Clinton has been advocating. And I think she's right that the way to save civilian lives and reduce the number of refugees is shut down air traffic in the northern part of Syrian.

I think, Judy, that's an idea that the administration has to consider now, given these events.

JUDY WOODRUFF: And we know a number of candidates in fact are talking about that. Well, the situation only got more complicated today.

Nicholas Burns, Angela Stent, we thank you.

NICHOLAS BURNS: Thank you.