Complex sentence: He remained active in politics, and from 1879 to 1881 served as Assistant Secretary of State, but afterwards remained in the private sector until 1897, when President McKinley, for whom Hay had been a major backer, made him Ambassador to the United Kingdom.
Split into simpler sentences: He remained active in politics, and from 1879 to 1881 served as Assistant Secretary of State. Hay afterwards remained in the private sector until 1897, when President McKinley, for whom he had been a major backer, made him Ambassador to the United Kingdom.