On a dusty training ground in Niger, U.S. Special Forces officers teach local troops to deal with suspects who resist arrest. "Speed, aggression, surprise!" an instructor barks as two Nigeriens wrestle a U.S. adviser out of a car.

The drill in the border town of Diffa is part of Operation Flintlock, a counterterrorism exercise for nations on the Sahara's southern flanks that the United States organizes each year. Washington's aim is to tackle Islamist militants in the Sahel region while keeping its military presence in Africa light.

A growing number of European nations taking part shows their increasing concern about security in West Africa. Central to the international effort is a blossoming relationship between the U.S. and France, the former colonial power and traditional "policeman" of the turbulent region.