The Office of the Historian is a wonderful site from the Department of State. It contains the text of many historical documents from the field of foreign affairs and diplomacy. The documents pertaining to Kennedy and the Cold War, and to Nixon and Vietnam are especially interesting areas. Documents can be browsed by administration or theme.
There is a section called Milestones that provides an overview of the history of U.S. international relations broken down into time periods. This is a very deep site with so much to explore!