When chief of staff Andrew Card knelt down and told George Bush “America is under attack” 15 years ago Sunday, the words he whispered in the president’s ear in a Florida classroom launched what was supposed to be a planned, orderly response to a national emergency. But what followed instead was chaos, a breakdown in communication and protocol that risked international conflict and could have made Sept. 11, 2001, a still bigger tragedy. There were live nukes on the tarmac at U.S. airbases, a failed communications system, and a security protocol for the president and his potential successors — the...