Secret Service Director Mark Sullivan announced his retirement Friday, bringing to a close a turbulent period for the law enforcement agency that included a South American prostitution scandal and a pair of White House gate-crashers. In nearly seven years as director, Sullivan had to answer serious questions from lawmakers on two occasions about his employees’ actions on the job and off. Last May, in testimony before Congress, Sullivan apologized for the conduct of Secret Service employees caught in a prostitution scandal in Colombia. Thirteen agents and officers were implicated after an agent argued with a prostitute over payment in a...