The Department is responsible for planning and implementing American foreign policy. The Secretary of State is the President’s chief advisor for foreign affairs. As such, records of the U.S. State Department are crucial starting points for research in U.S. foreign relations. Note that many documents, especially after World War II, are classified and only available through declassified document sets or FOIA requests, if at all. Over time other structures such as the Department of Defense, NSC and CIA took over some functions of the Department of State and thus kept their own records which are also important tools for studying foreign relations.
It is possible to search all of the below sets along with large collections of 19th century State Department Records and British foreign relations material by using the Gale Artemis database: http://hdl.library.upenn.edu/1017/115778
Additional U.S. State Department Records from the 1960s as well as Diplomatic Post Records, Military Intelligence Reports, Documents of the National Security Council, Selected CIA records, and National Security Files (the records of the President and special advisors) are available in History Vault: http://hdl.library.upenn.edu/1017/101485