The town of Konna lies along the eastern bank of the Niger River in central Mali, a semi-desert, speckled with thorn trees, that turns vibrantly green during the brief summer rains. For nearly a year, since rebel Tuaregs—the nomadic Berber people who live in the interior Sahara region of North Africa—and Islamic militants seized control of northern Mali, this settlement of 20,000 marked the limit of government-held territory. Five hundred troops in pickup trucks with mounted machine guns stood guard in the bush just north of the town. Beyond lay empty scrubland and a paved road to Timbuktu and Gao, the two main population centers under the jihadists’ control.

On Wednesday night, January 9, forty pickup trucks filled with Islamist fighters and heavy weaponry descended on Konna. Taken by surprise, government forces managed to repel the initial onslaught. Around midnight, however, another 150 armed jihadist vehicles arrived. A thousand fighters attacked the town’s defenders from three sides, using rocket-propelled grenades and large-caliber machine guns. After an eight-hour battle, the government lines broke. Hundreds of soldiers retreated in panic through the dirt streets of Konna, some of them stripping off their dark-green camouflage uniforms and begging locals for civilian clothes.

Ousmane Bah, a truck driver, watched the Islamists roll into town at 3:45 on Thursday afternoon. Dressed in desert khakis, they blew up a handful of military installations, and herded people to Konna’s mosques. A local street preacher who had joined the militants last year commanded them to gather the corpses of government troops. “Bury your dead dogs,” he told them. The jihadists ordered Konna’s imams to inform the people, Bah said, that “Sharia law is now introduced in Konna, and all women must be covered.”

On Friday morning, according to Bah, the chief jihadist arrived to claim his prize. Iyad Ag Ghali is a burly Tuareg whose black-bearded face is well known in the country. A former diplomat, smuggler, and hostage negotiator, Ghali had now taken on a new identity: the founder and commander of Ansar Dine, or Defenders of the Faith, a radical Islamist organization allied with al-Qaeda in the Islamic Maghreb, a force financed partly by the ransoming of Western hostages. “He was wearing a black turban, and a long blue robe,” Bah told me. “He gathered people together and declared that Ansar Dine and al-Qaeda would run things now.”

Until recently Mali, a nation of 15.8 million people in the Sahel—the arid belt that extends across North Africa—was widely viewed as a gentle if very poor democracy, a favorite of low- budget tourists and world music fans alike. The Festival in the Desert, a kind of African Woodstock in the dunes near Timbuktu, drew thousands of Western and local visitors every January. Timbuktu itself, in the last few years, underwent an unlikely renaissance as a cultural oasis in the Sahara, with half a dozen libraries that preserved a trove…