Foreign relations of the United States
A major source of primary material on US foreign relations is: Foreign relations of the United States (Department of State, United States of America. Washington : US G.P.O.)
"The Foreign Relations of the United States series presents the official documentary historical record of major US foreign policy decisions and significant diplomatic activity. The series, which is produced by the State Department's Office of the Historian, began in 1861 and now comprises more than 350 individual volumes. The volumes published over the last two decades increasingly contain declassified records from all the foreign affairs agencies." [from the State Dept. website]
On the history of FRUS, see also http://history.state.gov/historicaldocuments/frus-history.
There are several online versions of parts or all of this series:
Printed volumes are shelved in Firestone Library: Non Circulating (Fnc) KZ231 .U534
Some volumes from the 1950s and 1960s are also available on microfiche at:
Government Documents Collection (DOCS) Call Number: S 1.1
As classified documents are released to the public, they may appear in any of several databases or web sites. No single site or database provides complete coverage, so the diligent researcher will consult them all.
Declassified Documents Reference Service
(Post World War II-1970s) Indexing and full text access to a rich collection of declassified documents from various government agencies, including the White House, the CIA, the FBI, the State Department, and others covering events following World War II. The online database was preceded by a microfiche series:
DDRS retrospective collection [microform]
MICROFICHE Located at A-16-G, cabinet 68; guide at Firestone Library: Non Circulating (Fnc) Z1223 .Z7 D36
Digital National Security Archive
Access to a wealth of significant primary documents central to U.S. foreign and military policy since 1945. Comprised of 22 core collections; each contains a diverse range of policy documents including presidential directives, memos, diplomatic dispatches, meeting notes, independent reports, briefing papers, White House communications, email, confidential letters, and other secret material.
For a fascinating insight into declassified documents, see also the Redaction Archive.