Britain’s Conservative Party members chose Ms. Truss, the hawkish foreign secretary, over Rishi Sunak, the former chancellor of the Exchequer. She will replace Boris Johnson, who is stepping down after multiple scandals. It makes her Britain’s fourth prime minister in six years and third female leader, after Margaret Thatcher and Theresa May. Truss served in Mr. Johnson’s cabinet and was not part of the Tory rebellion that led to his departure, Truss was most recently foreign secretary,, Truss has made a remarkable political journey to the top of the Conservative Party. Raised in a left-wing family, with a father who was a mathematician and a mother who was a nurse and teacher, she was an active member of Britain’s centrist party, the Liberal Democrats, as a student at Oxford University, once calling for a vote to abolish the monarchy. Switching to the Tories after she graduated, Ms. Truss advanced through six ministerial posts under three Conservative prime ministers: Mr. Johnson, Mrs. May, and David Cameron. Like Mr. Cameron, she campaigned against Britain’s departure from the European Union in the 2016 referendum campaign, only to become a full-throated Brexiteer after the vote.