Various archives concerning the issues of daily life in East Prussia during World War II have been preserved in the State Archives in Olsztyn (APO). In the archival units, it is possible to find both information concerning both the German preparations for the war in the province and the course of the war not only in East Prussia, but also on the Western Front – in Belgium or France, the process of reconstruction of the destroyed East Prussian villages and towns and the means of commemorating the fallen soldiers fighting on both sides. The archives of the First World War and its consequences relate to many units. The intention of the author was to present the history of the First World War on the basis of archives assembled in the APO and in the Royal Calendar of Prussian Evangelicals in 1916 and in 1917, stored in the Research Centre in Olsztyn. The files stored in the APO are diverse and allow for a comprehensive understanding of issues relating to the course of this war, ranging from mobilization, military operations, Russian occupation, to the crimes and war crimes committed by the aggressor on the local civilian population. A separate aspect touches on the process of reconstruction of the province from the time of destruction and includes the care of war casualties.