SANA RESEMBLES A FORTRESS, not just in its architecture but in its geography. It is set on a high plateau, surrounded by arid, craggy mountains. At its heart is the Old City, a thicket of unearthly medieval towers and banded spires that stands out sharply in the dry desert air. This was the entire city until a few decades ago, its high walls locked every evening at dusk. Today Sana is a far more sprawling place, with Internet cafes and swarms of beat-up taxis and a sprinkling of adventure tourists. The Old City gates are mostly gone now, and although men still carry the traditional daggers known as jambiyas in their belts, they also wear blazers, often with cheap designer logos on their sleeves. Like other Arab capitals, it is full of policemen, and there are occasional checkpoints manned by bored-looking soldiers in camouflage uniforms.

But Yemen is different. Beneath the familiar Arab iconography, like pictures of the president that hang in every shop, there is a wildness about the place, a feeling that things might come apart at any moment. A narcotic haze descends on Yemen every afternoon, as men stuff their mouths with glossy khat leaves until their cheeks bulge and their eyes glaze over. Police officers sit down and ignore their posts, a green dribble running down their chins. Taxi drivers get lost and drive in circles, babbling into their cellphones. But if not for the opiate of khat, some say, all of Yemen - not just those areas of the south and north already smoldering with discontent - would explode into rebellion.

One morning in Sana, I discovered a crowd of people protesting in the stone courtyard outside the cabinet building. Many had shackle scars on their wrists and ankles. They came from an area called Jaashin, about 100 miles south of the capital. But some of them, I found, did not even know that Jaashin was in the Republic of Yemen. Their only real ruler was the local sheik, Muhammad Ahmed Mansour, who is, it turns out, a kind of latter-day Marquis de Sade. Mansour is also a poet, who earns extra license for his cruelties by writing florid odes to Yemen's president. Some pilgrims from Jaashin said they were imprisoned, shackled and beaten by the sheik - who maintains his own army and several prisons - after refusing to relinquish their property to him. I asked Ahmed Abdu Abdullah al-Haithami, a bent old farmer in a tattered green jacket, what country he was living in. He looked up at me with imploring eyes. "All I know is that God rules above, and the sheik rules here below," he said. All of this, I later learned, was documented by Yemeni lawyers, who have been working on behalf of the people of Jaashin for years to little effect. As one lawyer, Khaled al-Alansi, put it to me, "If you can't fight sheik Mansour, how can you possibly fight Al Qaeda?"

Two thousand years ago, the area east of Sana held one of the earth's most prosperous kingdoms, a lush agricultural region of spices and fruits, fed by irrigation canals from a vast man-made dam. The Romans called Yemen "Arabia Felix," or Happy Arabia. Today, the eastern region is an arid wasteland. Most people scrape by on less than $2 a day, even though they live atop Yemen's oil and gas fields. There are few ways to make a living other than smuggling, goat-herding and kidnapping. The region is also, chronically, a war zone. Tribal feuds have always been part of life here, but in recent years they have grown so common and so deadly that as much as a quarter of the population cannot go to school or work for fear of being killed. The feuds often devolve into battles with bands of raiders mowing down their rivals with machine-gun fire or launching mortars into a neighboring village. No one knows how many people die in these wars, but Khaled Fattah, a sociologist who has studied Yemen's tribes for years, told me that hundreds of victims a year is a conservative estimate.

Every time I drive out of Sana I get an ominous sense of going backward in time to a more lawless era. As the city's towers fade in the distance, the houses drop away into level desert and occasional piles of construction rubble. The traffic thins out and consists mostly of pickup trucks carrying tribesmen with patterned cloth kaffiyehs tied around their heads. You pass the first of several checkpoints, where skinny soldiers in ill-fitting uniforms warily circle the car, looking for weapons or kidnapping victims. You pass towering, desolate mountains of black and brown igneous rock. Once you're out of Sana province, there are virtually no signs of the Yemeni state. Every able-bodied man seems to carry an AK-47 rifle over his shoulder; it's not uncommon to see rocket-propelled-grenade launchers. Only the oil and gas fields, hidden behind wire fences and vigilantly watched over by the Yemeni military, seem to merit the government's attention.

Last year I expected to see at least a few government soldiers when I visited the ancient city of Shibam in Hadramawt, the vast eastern province where Osama bin Laden's father was born. A few months earlier, four South Korean tourists were blown up by a suicide bomber as they admired the view of Shibam from across the valley. I was a little nervous. "Don't worry," my guide said, patting my shoulder as we walked up to the ridge where the Koreans died. "Ever since the bombing they have put this place on high security." But when we got to the top of the ridge there was not a single soldier or policeman to be seen. We gazed out over the valley in silence. A sign stood nearby, showing a pair of binoculars and the words in English "Discover Islam." As we began to leave, my guide smiled broadly and gestured at the sign. "The Koreans - they discovered Islam," he said, giggling at his joke.

Even in the capital, law and order often mean less than they do in other Arab countries. One afternoon I was having tea with Abdulaziz al-Jifri when a shot rang out nearby. I thought nothing of it; it might have been a firecracker or someone testing a gun. We were in the safest area of the city, a neighborhood called Hadda, where rich Yemenis and foreign diplomats have built an enclave in recent decades. But Jifri got up from the cushion where he was sitting to go see what happened. He came back 15 minutes later with a look of surprise on his face. A friend of the family, a wealthy tribal figure, had been shot dead a block away. The victim, Jifri explained, was walking up to the gate of his home when someone apparently shot him once in the head. There were no witnesses and no one even bothered to call the police, who are so corrupt and incompetent that most people view them as useless.