HeinOnline: Foreign Relations of the United States (FRUS) is a comprehensive database containing diplomatic correspondence and documents spanning the years 1861-1980. The series is organized by presidency, beginning with President Lincoln and ending with President Carter, and is further organized by year and by nation or region of note.
Browsing these documents gives one a genuine sense of what successes, difficulties and troubles faced US diplomats in each respective time and place. This database collection brings an international dimension to major events in American history. It also includes a set of other related works, containing agreements, reference materials, and other documents relating to various topics of international relations.
An example is Armed Merchantmen, which discusses attitudes toward arming commercial vessels during World War One. Another example, by John Latane, examines the interaction of the US Constitution with agreements and transactions made abroad. This database is very helpful to anyone researching US foreign relations, or even to someone interested in the role of the US in a particular historical events. Start your research here.