NAIROBI, Kenya—Islamist militants’ deadly 18-hour siege of an upscale complex here jolts a key U.S. ally in the war on terrorism and raises questions over how the Somalia-based insurgents have survived more than a decade of international military campaigns.
On Tuesday afternoon, unmasked attackers carrying AK-47s and explosives shot their way into the 14 Riverside hotel and office complex in the capital—a favorite hangout for foreigners and upper-class Kenyans—detonated at least one suicide bomb and barricaded themselves in...