NAIROBI, Kenya » They have lost their leader, their ports, their checkpoints and their territory.

They have lost thousands of men and much of their money.

They do not have armored personnel carriers like Boko Haram’s. Or poppy fields like the Taliban’s. Or oil fields like the Islamic State’s.

In the pecking order of the world’s leading terrorist groups, al-Shabab militants, based in Somalia, operate on a shoestring budget. But as the attack on a Kenyan university last week showed, they have become proficient in something terrible: mass murder on the cheap.

In the past two years alone, bare-bones al-Shabab teams of young gunmen have struck across Kenya, at a mall, on buses, at a quarry, in a coastal village and last week at a university, where four militants with rudimentary assault rifles killed 142 students.

In all, they have slaughtered hundreds of people and shaken Kenya, an economic powerhouse and cornerstone of stability in this part of Africa, with just a few men and a handful of light weapons.

"I call it the dumbing down of terrorism," said Matt Bryden, a researcher in Nairobi who has been working on Somalia for more than 20 years. "They keep it simple. They’re lightly armed, highly disciplined and relatively well trained."

"They’ve definitely lost some of their major revenue flows," he added. "But they’ve managed to survive a lean season."

Despite a major international military effort in recent years to retake Somalia and push al-Shabab out of its strongholds, especially ports on the Somali coast, al-Shabab fighters are proving to be frighteningly resilient.

Analysts say they lead a grueling existence, moving constantly from threadbare village to threadbare village, living off the land in one of the poorest lands on Earth. It seems all the theories about how to stop them do not seem to be working.

The Obama administration is deeply worried that al-Shabab, one of most violent branches of al-Qaida, might strike on U.S. soil, and the strategy against it has been to eliminate its leaders and deny them sanctuaries where they can plot operations.

In conventional military terms, al-Shabab is losing. They have been routed from many areas, and are no longer able to rake in millions of dollars by shipping out mountains of charcoal or importing cars, as they did just a few years ago. Even in the small towns in Somalia they still control, al-Shabab fighters are not safe. They are relentlessly hunted — from above.

Their revered leader, Ahmed Abdi Godane, was killed last year in a U.S. airstrike, and other al-Shabab agents have been killed by drones.

The U.S. government has invested nearly $1 billion in this strategy. But al-Shabab attacks, as shown by the university massacre in Kenya, continue to grow in scope and ambition, raising the question: How can they be stopped?

"It’s not an easy game," said Stig Jarle Hansen, a Norwegian professor who has written a book about al-Shabab. "You have to have a people-centric strategy. You have to bring security to the villages in Somalia and stop corruption within the Kenyan security services. I can’t tell you how many times I’ve heard over the past five or six years, ‘The Shabab is dying, the Shabab is dying.’

"The Shabab is not dying," he said. "Case closed."

Al-Shabab’s endless evolution provides a daunting global lesson in the battle against extremists. It is not all about territory, as al-Shabab has shown with its latest attacks.

On the other side of the continent, Nigeria and its neighbors are fighting Boko Haram, retaking towns and villages in an effort to stop the group from dominating large stretches of Nigerian territory.

In Iraq, the government and its allies, backed by U.S. airstrikes, are battling the Islamic State for control of Tikrit. In Yemen, there are concerns that a feared branch of al-Qaida will consolidate even more territory and influence amid a chaotic and expanding civil war.

Al-Shabab fighters once aspired to rule Somalia, and nearly did. They eagerly fed off the bitterness and anger many Somalis felt toward an Ethiopian force that was occupying their country. (The United States had covertly supported the Ethiopian invasion).

From 2007-10, al-Shabab steadily tightened its grip on Somalia, at one point controlling more territory than any other al-Qaida franchise — a chunk the size of Denmark.

Hansen calls this period al-Shabab’s "golden age."

"They were doing something like state building," he said. "They were administering territory. They were delivering services," while bullwhipping women and suppressing the local population in their harsh interpretation of Islamic rule.

But al-Shabab commanders made the mistake of hubris, thinking they could defeat a much larger, better-armed African Union force in conventional warfare.

Al-Shabab lost hundreds of fighters in street battles in Mogadishu, the capital, in 2010. Many more defected. Analysts estimate that their army has dwindled to 3,000 fighters, at most, from about 7,000.

Al-Shabab lost its major port, Kismayo, and then minor ones like Brava. Al-Shabab’s leaders are now believed to be concentrated around Jilib, a small inland town tucked in the mangrove swamps near the southern Somali coast.

As the map of its territory has violently changed, so have its tactics. Al-Shabab used to detonate huge car bombs in Mogadishu that blackened large stretches of the city and killed hundreds of people. But car bombs are expensive. Analysts say al-Shabab probably does not have the cash anymore.

But there may be another reason. The countless civilians killed in Somalia were almost all Muslim (the country is almost exclusively Muslim) and the central leadership of al-Qaida scolded al-Shabab for slaughtering so many Muslims.

So al-Shabab fighters did something they have always been good at: They changed. Now al-Shabab fighters sort their victims at gunpoint. They let Muslims go and tell Christians to lie down, eyes closed.

At the Westgate mall in Nairobi, Kenya, they asked shoppers questions about Islam to separate Muslims from non-Muslims. They did it again in the attacks at the Mandera quarry, shooting many Christian workers in the back of the head, at close range. And last week al-Shabab spared Muslim students — most of the students at Garissa University College, where they struck, were from other parts of Kenya, the majority Christian.

The fighters at the school also seemed tactically proficient, managing to kill six security officers.

Bronwyn Bruton, deputy director of the Ansari Africa Center of the Atlantic Council, a research institute in Washington, said she suspected al-Shabab were "beginning to play with class distinctions."

"Westgate and, to a lesser extent, Garissa University College are both enclaves of privilege in a country where youths, especially Muslim youths, are frustrated by the lack of economic opportunity," she said. "Eventually, Shabab is going to have to find a way to connect with non-Somali Muslims."

Al-Shabab seems to pick many of its targets very carefully. In Nairobi, al-Shabab fighters did not attack just any mall; they attacked the glitziest, most expensive and most symbolic of Kenya’s prosperity and dreams.

Garissa University College, now closed, had one of the largest concentrations of non-Muslims in that part of Kenya. It was lightly guarded. And it lies relatively close to the Somali border, not far from the same areas in Somalia where al-Shabab agents still circulate.

In claiming responsibility for the attack, an al-Shabab spokesman said the university was part of Kenya’s "plan to spread their Christianity and infidelity" in a Muslim area that al-Shabab consider a "colony" under Christian control.

Several analysts say al-Shabab is now in fierce competition with the Islamic State for foreign recruits. Al-Shabab commanders cannot deny that their territory has shrunk, so killing large numbers of civilians is a way for them to stay relevant in the terrorist world.

"Stopping the Shabab is going to be tough," Bruton said, adding that the region’s security services desperately needed reform, and "that will take years."

Bryden, the Nairobi researcher, said al-Shabab had made enormous strides in recruiting youths in Kenya, Tanzania, Uganda and Djibouti, and many do not exactly fit the stereotype of marginalized or poor recruits.

On Sunday, Kenyan officials said that one of the gunmen from the attack on the university was a young, bright, privileged Kenyan who wore $200 suits and whose father was a local chief.

"The Shabab is becoming more decentralized," Bryden said. "That makes it more resilient to decapitation strikes."

Bryden, like several other analysts, does not believe firepower can destroy al-Shabab.

"There has to be a political vision across this region to tackle the Shabab," he said.

"Right now, that doesn’t exist."

Jeffrey Gettleman, New York Times

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