Historians often overlook a wealth of information on historical events published by the U.S. Government. Much of this material is available in libraries round the country in print form (Including the Alkek library) and, increasingly, in online formats.
Many government publications fit the definition of a primary source, as defined in the tab at the top of this page.
The Papers Relating to the Foreign Relations of the United States series is the official documentary historical record of major U.S. foreign policy decisions that have been declassified and edited for publication. The series is produced by the State Department's Office of the Historian and printed volumes are available from the Government Printing Office.The series begins with the administration of Abraham Lincoln in 1861. There are two cumulative indexes covering 1861-1899 and 1900-1918. The organization of the series is generally chronological. Each volume has a subject and author index. The title changes slightly over the years but, all volumes are on the 4th floor of the Alkek Library in the Government Information area at the call# in the link above.
The U.S. Serial Set contains a wealth of information on American diplomacy throughout history. See this guide for information on how to mine this wonderful resource.