GWEN IFILL: The discovery that one of the brothers involved in the Paris attacks received al-Qaida training in Yemen put that terrorist group and Yemen back in the spotlight.

Joining me for more on the threats from both, and the U.S. strategy to try to contain them, is chief foreign affairs correspondent Margaret Warner.

So, Margaret, do these attacks in Paris change the assessment here of what the risks are?

MARGARET WARNER: I don't think it changes the assessment, Gwen, because U.S. intelligence officials have been saying for some time that they thought the real danger wasn't so much returning foreign fighters, but at least as much either lone wolves or those inspired to attack.

And so one U.S. official told me today that he was really not surprised by what happened in Paris. As you know, the chief spokesman for Islamic State, this guy Al-Adnani, back in September called on followers in all these Western countries to attack their own targets at home.

And you saw some — attack in Canada. You saw attempted attacks at hostage-taking in Australia. So, from the U.S. perspective, the U.S. feels it has a pretty good bead on who has — the very few from U.S. who have tried to go over have gone to Iraq and Syria.

What they are most worried about, even though these two brothers were on the no-fly list, that there are plenty of others out there who are inspired, who can get into the United States without a visa to come from Western Europe.

GWEN IFILL: What is it about Yemen? You were there in 2010 on a reporting trip for us. And at the time, there was some question about whether it was kind of a hot bed for jihadism. And I wonder whether it is again.

MARGARET WARNER: Yes and yes.

I went immediately after the Christmas Day bomber to try to figure out why it is considered a hotbed. And I saw a very senior White House official who has an even more senior job today, whom I won't name, who said, Yemen is the greatest threat to American security out there.

And I was stunned, because that wasn't the thinking then in early 2010. The reasons, Gwen, are both historic and today. Historically, Yemenis have always kind of punched above their weight. When there's a call to jihad, they go. Many more than their numbers would suggest went to Afghanistan.

Then, what you had was the formation within Yemen of Saudi and Yemeni branches. They combined to create this al-Qaida in the Arabian Peninsula. The Saudis are very tough on people like that. The Yemenis do not control most of their own territory. So you have a permissive environment because you have vast ungoverned spaces. Then you had enter the phenomenon of al-Awlaki, this very effective American-born preacher, who also found haven there.

The Yemenis always give haven. And finally you had a lot of returning Guantanamo detainees who were Saudi who wouldn't have been allowed to exist in Saudi Arabia, but came to Yemen, and a very ineffective government that the U.S. was trying to help.

GWEN IFILL: But there's not — I wonder about the distinction anymore. Al-Qaida on the Arabian Peninsula is different, in theory, from the Islamic State group.

MARGARET WARNER: Yes.

GWEN IFILL: We heard one of the suspects say he was representing al-Qaida and the other said he was representing I.S. Is that a distinction without a difference anymore?

MARGARET WARNER: One intelligence official said to me he thinks it's 50 percent collaboration and 50 percent competition. No doubt they're competing in a place like Syria, in which the Jabhat al-Nusra, which is the Al-Qaida group, happens to be fighting with Western-backed rebels against President Assad.

But the fact is, they share the exact same aims. And it's anti-Western and it's anti-governments, local governments who are, they consider, too Western or too allied with the West. And so in that sense, the concern actually of intelligence officials is that there's been too much made of this separation and that even in the inner circles of the intelligence community, you start with groups and you keep analyzing them in groups in these stovepipes and you miss the connections.

GWEN IFILL: Final question for you. There has been much kerfuffle about whether the president should have gone to this big march in Paris over the weekend. And today the White House acknowledged, yes, maybe someone more high-ranking, if not the president himself, should have gone.

Is this something which has been of concern in Europe?

MARGARET WARNER: Well, talking to a senior Britain official today and others, of course they don't want to criticize the president, and the intelligence cooperation the U.S. is giving is huge. So, on an operational level, there is no complaint.

But, certainly, on the symbolic level, you can always count on the tabloids in Britain. The Daily Mail said, “Obama Snubs Historic Paris Rally.”

GWEN IFILL: Well, the tabloids here as well.

MARGARET WARNER: Yes.

GWEN IFILL: Yes.

OK. Margaret Warner, as always, thank you.

MARGARET WARNER: And a pleasure, as always.