President Barack Obama had hoped for a quick, convincing strike on Syria, but growing opposition and Great Britain’s stunning rejection of the attack has thrust him into the uncomfortable position of go-it-alone hawk. Just how Obama, whose career sprung from the ashes of George W. Bush’s Iraq policy, got to this extraordinary moment in his presidency is a tale of good intentions, seat-of-the-pants planning and, above all, how a cautious commander-in-chief became imprisoned by a promise. Obama seems likely to bull ahead with air attacks despite an impact and popularity that will be, at best, limited — an unsavory outcome...