The Main Collection of the Library supports research in all aspects of international affairs since the beginning of the twentieth century, and in the political, economic and social history of several areas of the modern world.
Languages represented include Western, Slavic, African and Middle Eastern languages.
The Collection contains over a million items, including books, newspapers, journals, pamphlets, publications of private organizations, microforms, and government documents. Specialized collections of rare and sometimes unique materials focus on subject areas within the general collecting fields. The foundation of the Main Collection was laid in 1919 at the Paris Peace Conference, when representatives of the fledgling Hoover War Library acquired materials from participating countries, previously unobtainable government documents and other items related to the various revolutions of 1917-1919, and materials on the new states established by the Conference. By taking this initiative in bringing together documentation on political, economic and social developments leading to the outbreak of World War I, as well as materials on all aspects of the war and its aftermath, the Library initiated its vigorous and sustained program of acquisitions that continues today. Between the two world wars and during World War II, the Library covered practically all European countries and all revolutionary movements.