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.
Access to significant declassified primary source U.S. government documents central to U.S. history, policy, politics, and international relations since 1945. Each collection 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.
1945-present.
"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]
There are several online versions of parts or all of this series:
FOIA amendments signed into law in 1994 added a requirement that agencies must establish an Electronic FOIA (EFOIA) Reading Room. The links below take you to FOIA Reading Rooms for several related government agencies.