JOHANNESBURG, South Africa— When a small group of Nigerian Islamist militants attacked two police stations and killed an officer, government security forces moved in swiftly, announcing they had “crushed” the revolt.

Ten months later, the militants attacked two more stations, slaying four officers. This time Nigerian security forces killed dozens of fighters, and the state police commissioner announced that it was just “a matter of hours before (all) the militants will be flushed out.”

More than a decade later, that hasn’t happened.

Instead, the northern Nigerian group known as Boko Haram, in response to the government crackdown, has grown exponentially and is increasingly targeting civilians. In the last week alone, it has been blamed for a suicide bombing at a bus station in the capital, Abuja, in which 71 civilians were killed, and the abduction of more than 230 teenage schoolgirls from a northern village. The girls remain missing.

On Wednesday, rumors on social media that the militants had seized a highway in southern Nigeria caused chaos and panic as security forces raced to the area. Since January, a quarter of a million Nigerians have fled their homes.

The attacks come at a particularly embarrassing moment for Nigeria, only weeks before it is to host an annual World Economic Forum on Africa, with hundreds of delegates from around the globe set to fly in to a nation that has just emerged as Africa’s biggest economy, surpassing South Africa.

The campaign to destroy Boko Haram has turned Nigeria’s impoverished, predominantly Muslim northeastern desert region into a war zone, as the fighters repeatedly change tactics and up the ante: urban assaults in 2009; suicide bombings, assassinations and drive-by shootings in 2010 and 2011; devastating mass attacks on schools, barracks and unprotected northern villages more recently.

Thousands of Nigerians have died, schools and colleges have been closed, and the region’s economy has been ripped apart.

Analysts say a key reason for the failure to eradicate the group is that it is nearly impossible for an army to use brute force alone to fight indigenous guerrillas who can melt away in a vast desert region.

Moreover, they say, reliance on brute strength doesn’t create the conditions for winning hearts and minds in the nation, which is divided between the marginalized mainly Muslim north and the largely Christian south.

“The Nigerians have long used one instrument, which is a very blunt instrument: military force,” said J. Peter Pham, director of the Africa Center at the Washington-based Atlantic Council think tank. “You also need intelligence,” he said.

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