Erik Prince, a private military contractor and prominent supporter of President Trump, is working with Haiti’s government to conduct lethal operations against gangs that are terrorizing the nation and threatening to take over its capital.Mr. Prince, the founder of Blackwater Worldwide, signed a contract to take on the criminal groups that have been killing civilians and seizing control of vast areas of territory, according to senior Haitian and American government officials and several other security experts familiar with Mr. Prince’s work in Haiti.Haiti’s government has hired American contractors, including Mr. Prince, in recent months to work on a secret task...