The U.S. Department of State is the executive branch agency most closely involved with the foreign policy and diplomacy of the USA. Its website is the best place to begin research on American foreign policy. Here you will find links to information about its activities and publications, as well as, those of its subagencies.
"The Foreign Relations of the United States (FRUS) series presents the official documentary historical record of major U.S. foreign policy decisions. The series began in 1861 and now comprises more than 400 individual volumes. The FRUS series provides an indispensible resource for American citizens and others around the world who seek to understand U.S. foreign policy and strategic planning, international relations, economic affairs, and transnational social and cultural developments. The Foreign Relations series represents a longstanding effort to inform the public about governmental decision making while also protecting essential national security interests."
Foreign Relations of the United States, 1861-1949. (Library has microfiche to 1949; book holdings 1933 – 1976. Because of the "thirty year closure" rule, scattered volumes have appeared for events after 1957. No volume was issued for 1869.) Includes communications both to and from foreign government and their representatives in foreign countries with officials in the Department of State. Volumes published prior to 1925 do not usually contain documentation relating to the negotiation of treaties. As correspondence relating to a particular year may be withheld for the time being and appear in a later volume, it is advisable to consult the subsequent volumes in the series. First volume of microfiche set includes general index to all 56 volumes by country. Supplements in microfiche include Lansing Papers; material on Japan, China, and the Soviet Union from the 1930’s and 1940’s; and Current Economic Developments, 1945-54 (which is indexed in paper).
The two congressional committees most involved with U.S. foreign policy are The House of Representatives Committee on Foreign Affairs (known as the Committee on International Relations from 1995 until 2007) and The Senate Committee on Foreign Policy. Each of these committees issues hearings, committee prints, and miscellaneous publications dealing with foreign policy.