The killing of four American special operations soldiers in Niger has highlighted the increasing role elite units are playing across Africa, which is rapidly becoming a major center of US military action. Their mission is to counter the advances of a slew of jihadist movements across the continent, including Al-Shabaab in Somalia, affiliates of the Islamic State group in the Sahel region and Boko Haram in Nigeria. Of the 8,000 special forces "operators" deployed globally this year, more than 1,300 are in Africa, according to officials from the US Special Operations Command (SOCOM), which is based in Tampa, Florida. Another 5,000 or so are in the Middle East. In five years, the number of US commandos in Africa has tripled...