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The latest conquest was the fishing town of Baga on the shores of Lake Chad, which fell to the Islamists last Wednesday.

“For five kilometres, I kept stepping on dead bodies until I reached Malam Karanti village, which was also deserted and burnt,” one surviving fisherman, Yanaye Grema, said. Boko Haram’s fighters have now achieved mastery over 11 local government areas with a total population exceeding 1.7 million, according to the official 2006 census.

Once, the movement’s fighters would launch hit-and-run attacks on defenceless villages. Now, Boko Haram’s realm stretches from the Mandara Mountains on the eastern border with Cameroon to Lake Chad in the north and the Yedseram river in the west.

The Nigerian army, crippled by corruption and incompetence, has shown itself unable to resist the jihadist advance.

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Last September, Abubakar Shekau, the self-styled “Emir” of Boko Haram, proclaimed his ambition to conquer a “Caliphate” and follow the example of Islamic State of Iraq and Al-Sham.

Diplomats believe this was a logical escalation of Boko Haram’s campaign.

“There is a copycat element at work here,” said Andrew Pocock, the British High Commissioner to Nigeria. “If ISIL can declare a Caliphate, then so can we. Boko Haram want to be seen by their peers as grown-up jihadists. They want to show ’we can control territory, we can control a Caliphate’.”

There is also a clear practical rationale for Boko Haram to capture territory. “Success, and they have had success, creates a different kind of requirement,” added Mr. Pocock. “You need a place where you can base yourself and keep equipment and supplies and, indeed, captives. It means that you’ve got to hold territory.”