When President George W. Bush asked Colin Powell if the United States should invade Iraq in 2003 and overthrow dictator Saddam Hussein, the secretary of state and former general refused to give a straight answer. “I didn’t say to him, ‘I oppose this war,’” Powell says in a book published today about U.S. foreign policy after the Cold War. “Because it wouldn’t have worked.” That admission sheds light on Bush’s decision to launch the war that defined his presidency. Powell acknowledged his muted opposition in James Mann’s new book, The Great Rift, an account of how Powell’s rivalry with Vice...