PORT-AU-PRINCE, Haiti — A first contingent of 400 Kenyan police officers arrived in Haiti on Tuesday morning, more than 18 months after then-Prime Minister Ariel Henry first asked the United Nations to send a security force to restore order to this violence-racked nation.

       The goal of the force: to wrest the country back from the heavily armed gangs that control more than 80 percent of the capital, inflicting some of the worst violence it has seen in decades, and to allow authorities to hold new elections for president and the National Assembly.