MICHEL MARTIN, HOST:

We wanted to hear more about the group that's claimed responsibility for the attack, in fact has boasted about it. The group is called al-Shabab, which means "the youth" in Arabic. It's based in Somalia where it once controlled the Somali capital, Mogadishu, and a major port before being ousted by African Union forces. On Saturday, al-Shabab took to Twitter. One tweet said, quote, the attack at Westgate Mall is just a very tiny fraction of what Muslims in Somalia experience at the hands of Kenyan invaders. We wanted to hear more about this, so we've called on Mary Harper. She's the author of "Getting Somalia Wrong?: Faith, War and Hope in a Shattered State." She's covered the country for the BBC for many years. Mary Harper, welcome back. Thanks so much for joining us once again.

MARY HARPER: Thank you for having me.

MARTIN: What do you think is most important to know about the group in relation to this attack?

HARPER: I think the most important thing to know is that even though, probably, your listeners will have heard lots of media reports that al-Shabab, which is a group linked to al-Qaida, is very much on the back foot in Somalia, the evidence on the ground, and now in Kenya, points otherwise. It may have been pushed out of major towns, as you mentioned, but it still controls the majority of southern and central Somalia, and it still has the capacity to strike not just in Somalia, which it does regularly, but also across Somalia's border, right in central Nairobi this spectacular, dramatic attack, which it staged on Saturday.

MARTIN: Does this represent an escalation in their capability, to your knowledge? We understand that they're still mounting suicide attacks often in Somalia, but does this attack signal something new about the group, either in their abilities, their capabilities, their intentions, their ambitions?

HARPER: I think what it tells us is that since al-Shabab was losing ground in Somalia, it basically looked outwards, towards the region, and, for example, it has been recruiting people in Kenya for a couple of years. It even has a website in basically the lingua franca of Kenya, Kiswahili. So it's been working very, very hard to broaden its membership, and this is perhaps evidence that it has indeed managed to do just that, but it's become a regional rather than a nationalist movement. So I think that that is the biggest lesson that is to be learned from the attack on Westgate.

MARTIN: What does that tweet that I read mean? Could you interpret that a bit more for us? The attack at Westgate Mall is just a very tiny fraction of what Muslims in Somalia experience at the hands of Kenyan invaders. What are they saying? And also, what do you draw from the fact that they were tweeting throughout the attack?

HARPER: Yes. I'll answer the first part of the question first. The reference to what Somalis have been suffering at the hands of Kenyans is because almost exactly two years ago, in October 2011, Kenyan forces invaded Somalia. This was the first time Kenya had ever invaded a foreign country, and they basically - Kenya's been terrified that the extreme Islamism in Somalia will come down and infect Kenya, so they wanted to establish a buffer zone. So they basically took a war to al-Shabab, and al-Shabab is now saying, well, you know, Kenya declared war on us, so now we're going to beat back at Kenya, right at the very heart of Kenya, in this shopping mall. So they basically see themselves as a group that has been forced, I suppose, to act in this kind of way to Kenya because it sees Kenya as one of the absolute, principal enemies.

And talking about Twitter, al-Shabaab has got a fantastically sophisticated propaganda machine. It has sophisticated, slick propaganda videos, which it uses to seduce disaffected youth all over the world, including in America. Many Somalis based in places like Minnesota have gone back to fight jihad in Somalia. So Twitter is one of its - it's very, very media-savvy, and it has these very witty, quick-off-the-mark people who write its tweets in English for, basically for maximum media impact.

MARTIN: What is their global ambition? What is their larger ambition? I mean, they describe themselves very much as kind of defending what they see as their rights and territory and the rights of Muslims. How do they - what is their broader goal?

HARPER: You've brought up a very important point here. A few months ago, al-Shabab basically imploded - that there were two factions. One faction was very, very keen on keeping this holy war inside Somalia itself, not going beyond Somali borders. But the faction that won through was the faction that believed in a global jihad. The leader, whose name is Godane, has said that he wants to take jihad all the way to Alaska. He has global ambitions. And as that is the part of al-Shabab that is now dominant, it was perhaps - at least it's not surprising that one of their first - their first major attack, following that sort of splitting apart that it had, was in a foreign country.

MARTIN: How does this group sustain itself given that, as you said, it's been the subject of very aggressive efforts by African Union forces, Ethiopian, Kenyan forces? It's been considered a terrorist group throughout. I mean, we have about a minute left. How does it continue to sustain itself?

HARPER: Yes, it has been increasingly difficult for al-Shabab to sustain itself because it lost, as you mentioned, its control of this major port where it was getting a lot of revenue from selling charcoal and also, it lost a main market in Mogadishu. So now it relies on donations from overseas, and also, it's become, I think, a smaller movement. But it doesn't take very many people to stage an attack like the one in Nairobi because if you're prepared to die for your cause, you only need a handful of fighters to carry out these acts that have dramatic, huge and devastating effect.

MARTIN: Mary Harper is the BBC's Africa editor and author of "Getting Somalia Wrong?: Faith, War and Hope in a Shattered State." We caught up with her in London. Mary Harper, thank you so much for speaking with us.

HARPER: Thank you. Transcript provided by NPR, Copyright NPR.