On Saturday, September 21, 2013, Camille Cusumano had just hit the send button on her computer that would alert thousands of her contacts she was headed to Nairobi to teach tango to slum-dwelling youth when a phone call came informing her that Somali terrorists had bombed the Westgate Mall in Nairobi. Some 67 people died and more than 175 were wounded. The Islamist group al-Shaabab claimed responsibility. She wondered what al-Shaabab thought of the...