porphyrogenitusIt’s hard to recommend *one* primary source for all of American foreign policy (primary sources being sources of the original actors and original/contemporary reports, so they are rarely a comprehensive overview of any subject. The only time a primary source would be an overview is if you wanted to know what so-and-so’s view of American foreign policy was, and read all the documents written by that person; i.e. “what was Murray Rothbard’s interpretation of American foreign policy? – the primary documents would then be all Rothbard’s writings and recorded speeches, interviews, and lectures. Secondary sources would be things written by other people about Rothbard’s views on American foreign policy). Here for example is just one selection of links to primary documents covering one 16 year period.
It might help if you narrowed it down to a specific topic or period in American foreign policy.