The library has many sources that may be of interest which cover our first century of diplomacy.
In addition to basic keyword searches, subjuct headings are often helpful. Try searching by subject-
This will bring up a lot of subjects, scroll down just a bit to see the subjects broken out by particular years. For example,
United States Foreign Relations 1815 1861
United States Foreign Relations 1775 1783
Also look for subjects dealing with specific countries. For example: United States Foreign Relations-France
Always look for subject headings that end with the word "sources" or "historiography". These types of books wil often be primary sources or provide bibliographies or other information about where specific sources can be located for your research.