The gunmen stormed in just as dawn broke over the school in a remote village in northeastern Nigeria. There were around two dozen of them, and, survivors later recounted, they worked quickly, methodically and with unflinching brutality.

"Allahu akbar," they shouted as they lined up students and murdered them with single bullets to the head. Some of the teenage pupils were burned alive when their dormitories were locked, doused in gasoline and set alight; those trying to escape were knifed to death.

They killed 46 boys all in all. Unlike the abduction of more than 200 girls from a school in the town of Chibok last month, in this attack they spared the girls and killed all the boys. The atrocity barely registered in the international headlines. That was almost a year ago, in July 2013.