The Secretary of State, appointed by the President of the United States with the advice and consent of the Senate, is the President's chief foreign affairs adviser. The Secretary carries out the President's foreign policies through the State Department and the Foreign Service of the United States.
In addition, the Secretary of State retains domestic responsibilities that Congress entrusted to the State Department in 1789. These include the custody of the Great Seal of the United States, the preparation of certain presidential proclamations, the publication of treaties and international acts as well as the official record of the foreign relations of the United States, and the custody of certain original treaties and international agreements. The Secretary also serves as the channel of communication between the Federal Government and the States on the extradition of fugitives to or from foreign countries.
The Secretary is the highest-ranking member of the President's Cabinet. Under the Presidential Succession Act of 1947, he or she is fourth in the line of presidential succession; this amended law amended the 1886 Act which had placed the Secretary of State at second in line.
Only two Secretaries are identified:
Cordell Hull served as Secretary of State when the Race invaded Earth in 1942. In that capacity, he frequently treated with the USA's allies and with the Race. In 1944, Hull became President of the United States after Franklin D. Roosevelt died unexpectedly; Vice President Henry Wallace had been killed the year before. Hull appointed General George Marshall as his successor. Marshall participated in the Peace of Cairo, which ended the fighting.
Other Secretaries of State
In addition to the above, Harry Turtledove has written alternate history and science fiction stories in which the Secretary of State plays a prominent role or, at a minimum, in which the incumbent Secretary of State is referenced.
William Seward is the incumbent Secretary of State in The Guns of the South. After the Confederate States wins the Second American Revolution in 1864, Seward is one of three U.S. representatives who negotiate a peace treaty.
James Byrnes is the incumbent Secretary of State under President Harry Truman in The Man With the Iron Heart, where the actions of the German Freedom Front make his term far more difficult and contentious than it was in OTL.
Dean Acheson holds the position in the 1950s in both Joe Steele and The Hot War series. Acheson plays a minor role both works. In the former, he serves first under President Joe Steele, then under Steele's successor, John Nance Garner. In the latter, he serves under President Harry Truman as in OTL, but his term includes World War III. In both works, Acheson meets an untimely end, dying in a plane crash in Joe Steele, and presumably dying in the atomic bombing of Washington, DC.
Sec. Jackson is the Secretary of State in 2117 in the short science fiction work "Getting Real". He first tries to initiate an end to the "Real" trade with the Chinese government, then winds up negotiating an end to the disastrous war that follows.
In the short science fiction work "Vilcabamba", the office of Secretary of State becomes the Secretary of Alien Affairs after the alien Krolp invade Earth. The unnamed incumbent advises President Harris Moffatt III on his plans to resist the Krolp.
Historical Secretaries in Non-Secretarial Roles
Several historical Secretaries of States have appeared in the works of Harry Turtledove in a capacity other than as Secretary of State.
In an alternate where the U.S. did not adopt the Constitution and dissolved shortly after, John Quincy Adams became the head of state of the independent country of Massachusetts, but almost certainly not Secretary of State of the United States.
Dean Rusk receives a contemporary reference in The Hot War series. It's possible he's killed in the atomic bombing of Washington, DC. The series ends on January 1, 1953, seven years before he became Secretary of State in OTL.