Mild mannered and studiously polite in person, Mr. Frost was nonetheless the consummate Brexit hard-liner, a man who believed that only a combative approach — and a willingness to walk away without a deal — would win concessions in talks with the European Union.
He was a leading architect of Britain’s split with the bloc, negotiating a basic trade deal that took Britain out of its economic structures. He was such a close ally of Mr. Johnson that the prime minister delighted in calling him “the greatest Frost since the Great Frost of 1709,” referring to a particularly brutal cold snap more than three centuries ago. But he was not liked in Brussels, where there appeared to be little sadness about his exit.
Ms. Truss is a belated enthusiast for Brexit; she campaigned against it in the 2016 referendum. As foreign secretary, a position she assumed in a September reshuffle of the cabinet, she has pushed for new partnerships for Britain in Asia and the Pacific, more trade deals outside Europe and a tough line on Russia and China.
She does not have the same credentials among Brexit hard-liners as Mr. Frost did, and some critics questioned whether she was qualified to be foreign secretary. But conservative Britons appear to have few qualms: According to one recent survey of Conservative Party members, she was ranked as the most popular cabinet member.
Northern Ireland is a particularly fraught and high-stakes brief for Ms. Truss to inherit. Britain has threatened to suspend part of an agreement that Mr. Johnson himself negotiated, known as the Northern Ireland protocol.