The global impact and the disastrous consequences of the Great War were what prompted Herbert Hoover in April 1919 to initiate his collection of contemporary and historical documents to enable the study of war, revolution, and peace. Since World War I had started and was fought mostly on the European continent, materials from European countries, many of them ephemeral, initially formed the center of Hoover's collection efforts. In the decades that followed, the collection expanded greatly geographically and thematically, yet developments in Europe remained a core interest. Today, European materials still constitute the largest and most comprehensive part of Hoover’s international holdings.