BAMAKO, Mali—“The terrorists are quick,” German Chancellor Angela Merkel told reporters after a summit with the leaders of Mauritania, Mali, Niger, Chad and Burkina Faso in Ouagadougou in May. “This is why we have to be quicker, so that we can beat them.”

What happens in the Sahel, the vast sub-Saharan region of Northern Africa, “is not only the responsibility of the region, but is also a European responsibility,” Merkel added in what was for her some uncharacteristic alarmism. “If chaos gains the upper hand here—something we want to prevent—other areas would be impacted.” ...