Subject Headings are terms or phrases that best summarize the overall contents of a work and provide access to its most important topics. To search by subject term, navigate to the Advanced Search, select "subject" as the drop-down option, and paste the appropriate subject term in the search box. Apply additional filters as needed.

A good starting place is searching using the broad subject phrase United States—Foreign relations. However, you can combine geographic terms and time periods with different subjects to find specific sources on your topic of interest. Insert a double dash to combine terms (--).
For example:
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United States—Foreign relations—Nicaragua
National security—United States
Diplomatic relations
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United States—Foreign relations—Treaties