NAIROBI, Kenya — THIS was the attack we all knew was coming.

The Westgate mall, which is owned by Israelis, was a glitzy mecca for rich Kenyans and expats, a symbol of Kenya’s newfound decadence. My wife and I went on dates there all the time, catching a movie in a theater as comfy as any in the United States and then dropping $100 for some sushi. After we had kids, we’d take them to Westgate for shopping and ice cream, and it’s where my son Apollo, born and raised in Kenya, rode his first escalator. Westgate, actually, was where I interviewed my first real live Somali pirate (by phone). He was bobbing on the bridge of a hijacked tanker in the middle of the Indian Ocean. I was sitting at a cafe, drinking a banana smoothie.

On Sept. 21, at 11:30 a.m., on a typically bright and pleasant Nairobi morning, Islamist militants with military-grade weaponry stormed into Westgate, turning it into an abattoir. The first of the more than 60 people who were gunned down were sitting at that same cafe where I used to do my interviews, and the steps that I used to trot up holding my son’s hand are now smeared with blood.

For the past seven years that I’ve lived in Kenya, I’ve been following two very different story lines that represent what’s happening in contemporary Africa and that collided that fateful day in Westgate. The first is of the dramatic expansion of Africa’s middle class, now more than 300 million people, and perhaps there’s no better place on the continent to watch this than in Nairobi, where new office blocks are rising above the tin-shack slums, new bistros are popping up all over the place and taxi drivers are getting on Facebook. It’s essentially Africa joining the world.

When I first came here more than 20 years ago, the difference between life in the States and life in Kenya was enormous. There were barely any malls, for instance, and they sold things like yellow plastic jerrycans and roughly machined pots. Now you can get everything here — the latest Macs, frozen yogurt, Old El Paso taco mix — and at Westgate, it was all under one roof.

But at the same time that I’ve been chronicling this rapid, almost dizzying development, I’ve become a specialist in despair. Sub-Saharan Africa is still home to some of the poorest, most violent countries on earth: the Democratic Republic of Congo, Somalia, South Sudan, the Central African Republic — places where the government is a ghost and civilians are stalked, raped and killed by men with guns. You have no idea how many guns are in these places, and at the end of the day, all that separates Kenya from Somalia, the promising and the broken, is a thin line in the desert that usually goes unpatrolled. To live the good life here, to take the kids to the water park and hang out in new wine bars while a medieval mix of famine, plagues, warlords, pirates and sudden death seethed next door, did seem too good to be true.

And we had been warned. One weekend, as my wife and I were strapping the kids into the car, about to head out for the week’s shopping, a friend from the American Embassy called to say there had been a terrorist alert (another one) and to stay away from Village Market, another nice mall.

It might sound as if we go to malls a lot. We do. Nairobi doesn’t have so many public parks or playgrounds or areas to stroll. People meet up in malls. On Saturday mornings, families flock to the malls.

The Shabab, the Somali militant group that has taken credit for the massacre at Westgate, knew this. They knew they were going to kill children, and according to witnesses, some of the assailants lifted their rifles and deliberately shot down toddlers.

I actually know some Shabab commanders from my first visit to Somalia in 2006, when the Shabab practically operated in the open. One night, a top commander named Abu Monsur, now one of the most wanted men in East Africa, came to my hotel to give me a beautiful Koran, trimmed in gold leaf. With a tear falling from his eye, Abu Monsur clasped both my hands and asked me to convert to Islam. He was passionate, devout and deadly serious.

Back then, the Shabab were Islamists, and they were militants, but they were also nationalists, hoping to rule Somalia, and many Somalis liked them for running out the warlords who had kept their country steeped in anarchy for 15 years. But the Shabab were also colluding with Qaeda terrorists, and over the past few years, as the United States bankrolled a proxy force of African armies, including Kenya’s, to destroy them, the Shabab became flamboyantly brutal.

They stepped up amputations, beheadings and stonings. A Somali girl told me, in barely audible puffs of whispers, how the Shabab had buried one of her friends up to her neck in sand, and then bashed in her brain with rocks. The Shabab didn’t care anymore. They were like a cornered animal. Even Osama bin Laden said it was too much.

I had this sinking feeling that when the Shabab finally gave up any hope of ruling Somalia, they would strike Nairobi. They had been reluctant before, because Nairobi was their back office, where the white-collar Shabab lived, the accountants, financiers and logisticians. The instant they hit here, the Shabab leaders knew, Nairobi’s Somali community would fall under such intense pressure and scrutiny that it would be impossible to do business.

But the way they struck — the scale, the organization, the drawn-out siege, the ruthlessness — was especially terrorizing. Most of us expected a big, indiscriminate bomb. Instead, the Shabab sent a death squad of 10 to 15 shooters, who stalked men, women and children, executing them one by one as they curled up in various corners of the mall and begged for mercy. Some Muslims identified themselves and were spared.

Everyone in Kenya has relived this terror to some degree, visualizing what it would have been like to be spread over your children or squeezing your wife’s hand as the killers’ footsteps came closer. One couple was found intertwined on the floor, together to the end.

KENYA can absorb a single disaster. In 1998, Al Qaeda simultaneously blew up the American Embassies here and in Tanzania, a harbinger of the 9/11 attacks, and Kenya stumbled but it didn’t fall. People rallied together, changes were made and confidence was restored. I saw this happen again after a dreadful election in 2007 when more than 1,000 people were killed in ethnic clashes. Dead people were strewn across Nairobi’s streets and thick black smoke churned up from the slums. I could smell the whiff of char from my own doorstep, just as I did this week, as Kenyan soldiers fought for three days inside Westgate to kill the last assailants who had holed up in a supermarket. But what additionally unnerves us about this attack is that some of the killers may have escaped in the mayhem, dropping their guns, changing their clothes and simply walking away.

If there is another major terror attack here, it will be devastating. Kenya will be branded as insecure and expatriates will leave in droves. The billion-dollar tourism industry will crash, and everyone from pilots to safari guides to the maids at the wildlife lodges will be jobless. Tourists eager to see spectacular game and life-changing vistas will go to other African countries, and thousands of Kenyans will go hungry.

One thing that worries me is that while the Shabab have proved themselves a ruthless, wily enemy, Kenya has a profound disadvantage: it has never invested in its public safety. Crime is rampant here, and police officers are badly paid (about $200 a month) and often deeply corrupt. There is no 911 to call, and even if there were, it might not have mattered last weekend because most officers do not have cars.

But that didn’t stop plainclothes cops from flocking to the mall, from all directions, at the sound of the first gunshots, and they didn’t huddle outside calling for backup. They charged right in, cheap pistols against belt-fed machine guns, and several were cut down, but they pressed on, running out of Westgate with terrified children in their arms. Someone just sent me a picture of Westgate’s supermarket, a macabre image of six bodies slumped behind a deli counter, gallons of blood sloshed across the floor. Those were shoppers, not employees, and they had been hunted down in their hiding place. If the police officers hadn’t dashed in, the death toll would have been in the hundreds.

Kenya is a very frightened place right now. But it is still a beautiful place to live. In the hours after the massacre, so many Kenyans lined up to donate blood that some were turned away. People here pull for one another. And right now, more than ever, that is what we need.

Gettleman is the East Africa bureau chief for The New York Times, currently on leave writing a memoir.