More on the way, but here are the links to the free online primary source collections I discuss in Appendix B of Americans and International Affairs to 1921:
HathiTrust Digital Library. This will work better if you sign in via your affiliated institution, but the open-access version is great, too.
Foreign Relations of the United States at the Department of State and a somewhat updated version of the pioneering original digitization project at the University of Wisconsin
Congressional Record from 1873 to 2016 and from 1995 to the present. Check your library’s holdings for access to its predecessors, The Debates and Proceedings in the Congress of the United States (1789-1824), the Register of Debates in Congress (1824-1837), and the Congressional Globe (1837-1873). See also the Biographical Directory of the United States Congress.
Library of Congress, including the Chronicling America: Historic American Newspapers collection
HarpWeek. This is the limited set of Harper’s Weekly cartoons and explanatory articles that are available to the public; in particular, look at the “Cartoon of the Day” section. Many college and university libraries have access to the full digital collection, and some have print copies as well.
Perry-Castañeda Library Map Collection
and a bonus map collection: the David Rumsey Historical Map Collection