Americans have long associated the fight against Islamist terrorism with the Middle East, as Sunday’s attack in the Turkish capital of Ankara reminds us. But that geographical horizon is also increasingly out of date. Witness the expanding grip of jihadists in Africa—and the Obama Administration’s belated but increasingly urgent attempts to fight it.

This is apparent from the March 5 air strike, by manned and unmanned U.S. planes, on an al Shabaab training camp in Somalia that killed an estimated 150 terrorists. Four days later...