The IHR's collection of primary source material and guides to sources covers all aspects of the First World War. The collections are arranged geographically, so there are sections on the war within relevant country collections, and in the thematic Military and International Relations collections. One of the strengths of the collection are the wide-ranging personal accounts, see below for some examples of the sources and themes that can be found within them. There are also strong holdings of national and international official sources.
A good starting point for research is the range of reference works, guides to sources and works on historiography.