Last week’s bloody melee between Iraqi police officers and the residents of the camp has not only raised fresh doubts in Washington about the worth of these assurances, but has also exposed just how little leverage American officials now have in a country they largely controlled for almost six years. It has also forced the Obama administration to confront some of the thorny issues that bedeviled its predecessor: how to prevent Iraq from falling deeper under Tehran’s influence, and how to fashion a tough Iran policy amid delicate negotiations to dismantle the country’s burgeoning nuclear program. Officials in Washington have...